Because I have been living outdoors for 7 years almost straight I am in the rare position to be able to report about the longivity of gear out of my own experience. I buy a new piece of equipment and use it until it completely breaks or fails - and after so many years I have a very good idea of what breaks when and why:
Tents: I have been using Tarptents through almost all my hiking career and I am extremely happy with them. I started with a Virga, then a Virga II followed by a Contrail and now I am using a Rainbow. After a couple of months of use the tent floor will get little holes, but this has never bothered me and I don't use a ground sheet either. The first thing of consequence that will break is the slider of the tent zipper. Depending on what sort of environment you are moving in (lots of sand or not) this will happen after 4 - 6 months of constant use. The slider will wear out and you will not be able to zip up the tent any more. This is a gradual process and you should act as soon as you experience the first difficulties. Of course this always happens in mosquito country where you depend on your tent being closed.... The problem is quite easy to fix with a needle and thread if you have spare sliders with you - don't leave for a long trip without the appropriate spare sliders and familiarize yourself with how to change a slider. I change sliders up to 3 times before the tent dies of old age! After about one year of use the tensioners for the guy lines will wear out and/or the guylines itself will become too slippery to tension them anymore. You can work around it by putting little knots in the guylines or using them with their maximum length, but it will be difficult to tension the tent then without restaking. This is not a life threatening problem but a sign that you should think of getting a new tent. Three times in my long outdoor career a tent pole has broken and I could always repair it with a repair sleeve. This usually happens when you do not insert the tent pole segments into each other correctly before bending them - they will then break at the thin segment end. Do carry a small repair sleeve on long trips! In my experience a Tarptent can easily withstand 1 - 1,5 years of constant (ab)use before it has to be replaced and this is an excellent life expectancy for an UL piece of equipment.
Sleeping pad: I use Therm-a-Rest Prolite inflatable sleeping pads of all sizes and thicknesses and no matter what you do they will delaminate sooner or later. Delamination means that the air chambers break and create a bigger and bigger bubble in your pad until it becomes too uncomfortable to use. It usually starts with two little chambers merging into one bigger and then gradually progresses. Act as quickly as possibly when this happens and replace the mat as you cannot stop the delamination process once it has started. It is a major quality problem that seems to concern all brands of pads that use perforated foam. It has nothing to do with strain or pressure put onto the mat. It will happen no matter whether you fold the pad for transport and storage or not. It has nothing to do with the age of the pad - only with the amount of time it has been used. I want to emphasise that this has happened to every single one of my sleeping pads and I had to exchange pads under warranty 8 times! The only reason why I am sticking with Therm-A-Rest is that this problem concerns all perforated foam mattresses, but with Therm-A-Rest at least you have a world wide life long warranty service. The maximum amount of time a Therm-a-Rest has survived constant use without delamination is 7 months. Once the delamation process has started you have anything from one day to 2 weeks before the bubble becomes too big to use it comfortably.
I have bought a NeoAir All Season for winter use and after only about a week I had a puncture which brought me into a dangerous situation in sub-freezing temperatures. On the other hand in my entire 7 year long outdoor career I have only had one leak in a Prolite pad....I have nevertheless continued to use the NeoAir on other trips and have always put a thin CCF pad or a thick groundsheet underneath it and have not had any more leaks. My NeoAir All Season has now been used 6 months and still no delamination so in this respect it is at least as durable as a Prolite.
Sleeping bag: I have been using Western Mountaineering sleeping bags for the first half of my outdoor career and depending on the climate I was alternating between the Summerlite, the Ultralite, the Versalite and the Puma. Generally speaking the bags are high quality and very durable. After about one year of constant use the slider of the zipper can wear out. As all WM bags have two sliders and usually only the top one that is most used wears out this is not a life-threatening problem. The slider can be replaced very easily if you have the appropriate spare. After about half a year of constant use the down in the bag will start clumping and thus reduce the warming capacity of the bag. Washing will restore part of the loft, but still it is my experience that no matter how and how often you wash the bags, after one year of constant use they will have reduced loft so much that the original temperature rating will not be achieved any more. After 2 years of constant use the down has so much deteriorated that the bag is practically useless and even washing will not restore it any more..
After a lot of problems with clumping down due to extended use and/or humid conditions I have now changed to synthetic quilts. I am using a BPL 240 quilt for 3 seasons and a Enlightened Prodigy 20 for colder temperatures. I have used both quilts for around 300 nights without any significant deterioration of loft or any other substantial defects. I realise that about every outdoor book and forum will tell you differently, but in my personal experience synthetic is much more durable than down.
Platypus water bottles: Like Therm-A-Rest they are another Cascade Designs product with a serious quality problem for long-term users. No matter what you do and how you treat a Platypus bottle it will start leaking after some months of use. Typical leaks will form at the mouth of the bottle, but I had leaks all over the bottles. Never ever trust a Platypus bottle - it will break! Always carry at least two bottles. If you are in a real pinch you can temporarily repair the leak with Seamgrip or Superglue. Superglue is better for repairs as it will not peel off like Seamgrip. If the hole is in a flat surface you can also use Gorilla Tape. Again I am only sticking to Platypus because they offer a life-time guarantee and can be replaced almost everywhere around the world. If you want a something bombproof, use Ortlieb water containers - they have survived 2 years of constant abuse and are still not leaking. They are way too heavy for UL hiking, but I use them for cycling and paddling.
Gas canister stoves: I started my hiking career with MSR Pocket Rocket stoves. After about 6 months of constant use the thread will wear out and you will not be able to screw the stove down to the canister any more. As long as the canister is completely full of gas the stove will still work on a low flame, but after a couple of days the gas pressure will be too low and your stove useless. In a pinch you can try to press the stove down onto the canister with a string/rope and wedges construction, but this is not a long-term solution. This has happened to me with two different Pocket Rocket stoves despite careful handling. MSR has refused to exchange them under warranty. I would never ever carry a Pocket Rocket on a long trip again and can only advise against them. I have now changed to a Snow Peak Giga Power and to my big surprise the thread is still holding up after almost 3 years of constant use. Apparently Snow Peak uses a better material for the thread than MSR. One last word on stoves with Piezo ignition: When I bought my first Snow Peak I wanted to give it back after a couple of weeks of use because the Piezo would not ignite the gas any more. Luckily the sales person explained and fixed the problem for me: The metal end of the Piezo where the spark comes out has to be at a certain angle towards the stove head. By just moving this metal rod a fraction of a millimeter the Piezo worked again.
Trekking poles: I have been using Leki trekking poles for my whole hiking life and they seem to be unbreakable. After about 3.000 to 5.000 km you will have to exchange the tips but this is about it. Always start a long trip with new tips or carry spares and you will have no other problem with Leki. One pair of my Leki poles has survived more than 15.000 km of hiking with no problem - great quality I can only highly recommend.
Leki has now introduced the speed lock system, an external locking system. Although the locking system itself works very well after about one year of use the whole system came off the pole. The plastic locking system is glued to the metal pole and after so many hiking days in rain and shine the glue had deteriorated and could not hold the plastic anymore. You can easily re-glue it with Superglue and the repair will hold for some months because the natural pole movement moves the locking system onto the pole.
Backpack: I started with Golite Gust packs and they survive about 1,5 years of constant abuse before you will get holes on the bottom from abrasion. A fantastic pack with a great life expectancy that unfortunately has been discontinued. I then changed to the Gossamer Gear G4 and now I am equally happy. The G4 has a lot of sewing on it and after about 4 - 6 months of constant use the sewing will break because the yarn is wearing out. This is a gradual process and can easily be fixed by re-sewing the seams with dental floss. Always carry a needle and thread, preferably dental floss for that purpose. If you poke holes into the pack body repair them quickly as otherwise the tiny hole will rip further and further and eventually turn into a life-threatening huge tear. I use repair tape for that but I sew the tape on to prevent it from peeling off through dirt and moisture coming in from the edges. If you are not doing hardcore bushwhacking for months on end the G 4 has a life expectancy of about 12 - 16 months of constant use. Considering the weight and the moderate price of the pack this is an excellent value.
Rain jackets: I have tried both Goretex Paclite and eVent - and have been disappointed with both of them. I first used an awfully expensive Hagloefs Oz Paclite Pullover and after a couple of months of use it started leaking like a sieve. I returned it under warranty and got a brand new one, but after a couple of months the same thing happened again. I gave up on Paclite and changed to an Integral Designs eVent jacket. Everything was fine in the beginning, but after about three months of constant use it was leaking despite washing and ironing it. I am now back to cheap Tyvek rain gear - it might not be perfect either, but at least it is cheap.
GPS: On some trips I have used a Garmin GPS of the etrex series. The first GPS died after about 3 months of use just like that without any warning. From one moment to the other it could not be switched on again. As it was out of the warranty period I bought a new Garmin etrex - not realising that Garmin offers a flat-rate repair service. The brand new Garmin died after 2 months of use - all of a sudden the background lighting died rendering the device useless. Garmin replaced it under warranty, but still a broken GPS in the middle of nowhere created a lot of problems for me. I will now have to see how long this device will hold up. I don't know whether I have just had a streak of bad luck but I can only say: Do not rely on your GPS as your only navigational device! Always carry a backup in whatever form (maps, compass, cell phone GPS etc.)
Shoes: I have been using Keen trail runners on all my last hikes - either Voyager or Targhee. I am very happy with them and prefer them to the Merrell shoes I have been using before. Keen shoes have a longer life expectancy and last about 1.000 - 1.500 km before they have to be replaced. That means that on the trail I need new shoes every 4 - 6 weeks and this is a major factor on my monthly budget. Even if the sole still looks good, do change trail runners after a maximum of 1.500 km. After that the foam has been compressed to a degree where it does not have a cushioning effect any more and your feet will start hurting. Do not try to save money here: This can cause serious and long-lasting foot problems.
Clothes: Here is some good news: Clothes like T-shirts, fleece pullovers, trekking shirts and pants have a surprisingly long life expectancy. No matter whether you buy cheap no name/generic stuff or expensive brands outdoor clothes survive up to a year and more before they start falling apart. It does not pay off to invest a lot of money into brand stuff. Pants have a shorter life expectancy and survive only about 6 - 9 months before the zippers start wearing out or the fabric starts ripping. But if you have chosen a light colored (T-)shirt you will encounter a cosmetic problem first. After a couple of months of wear the shoulder straps of your backpack will start rubbing off onto your shirt discoloring it. No washing machine will be able to remove those discolorings and the shirt will always look dirty. Either chose a dark color or live with the discoloring.
Insulation jacket: I have used a Montbell Thermawrap jacket for 5 years before I had to bury it. The synthetic insulation was still decent, though degraded by several washes and hundreds of times being compressed into the little storage bag, but unfortunately the lower part of the zipper fabric had worn out so much that I could not thread it into the slider any more. I would definitely buy it again.
Socks: Socks have a much higher life expectancy than shoes and survive about two or three times as long as shoes. On a thruhike I have to change shoes about every 4 to 6 weeks, whereas socks survive upt to 4 months when using gaiters. Gaiters increase the life expectancy of socks a lot by keeping debris out of your shoes that wears out the socks (and the skin on your feet). My observations are based on wearing Wigwam Cool-lite socks in combination with gaiters. Surprisingly there are great differences in quality between hiking socks. Once I had to use different brands of hiking socks and they barely survived 2 months.
Gaiters: I have used the same pair of Simblissity LevaGaiters for almost 20,000 km of hiking although I had to repair them several times. After about 6 - 9 months of constant use the lace hook will break. The problem now is to find an appropriate replacement. You will usually some sort of hook in a sewing shop and you can then sew it on with dental floss. I have replaced the hooks on my Simblissity gaiters 4 times before I decided to bury them.
Pot: My Tatonko 1 l titanium pot is the oldest and longest surviving piece of gear I own. I have used the same pot for all my hiking career cooking in it over a thousand of times. It does not even look very battered now. After a couple of years the rubber insulation of the handles has come off but this is only a major flaw. If the pot handles are sticking out while cooking they will not heat up anyways and therefore you do not really need this insulation. Bottom line: a titanium pot is an investment that will almost last forever.
Spoon: I started out using a plastic spoon out of weight reasons. But Murphy's Law has struck several times and the plastic spoons always broke at the most inconvenient moment. Although this is not a life threatening situation it is a major annoyance. No matter what plastic spoon I bought they all broke sooner or later. I have now changed to a Sea to Summit Titanium spoon and have been happy ever since.
Wallet: My Simblissity UL wallet is my second oldest piece of gear and has accompanied my for over 5 years. I like it so much that I even use it in my non-outdoor life. Only now after 5 years of almost constant use some seams are coming off and the velcro is wearing out. This is a very well thought out and built piece of gear and I cannot recommend highly enough.
Outdoor watch: I have used a Suunto Vector for over 4 years for outdoor use and in normal life. Although the watch itself still worked fine I had to replace it after 4 years because of problems with the battery compartment. The battery is covered with a plastic cover that is held in place by 3 little plastic catches and waterproofed with a plastic O-Ring. After so many years of use the O-ring material deteriorated making it more and more difficult to close the battery cover. And while trying to force down the battery cover the plastic catches will break off eventually. This leads to two problems: The battery compartment cannot be closed properly any more and water can penetrate. Also the battery is not held firmly in place any more. As soon as it loses contact the watch will reset itself... leaving you in the middle of nowhere with no idea of what time it is. Change the O-Ring as soon as it deteriorates and always be very careful when closing and opening the battery cover.
Head lamps: Depending on how long and where I go I use one of my three Petzl headlamps: E-Lite, Tikka and Tikka Plus. Unfortunately, the construction of the plastic casing lets it easily break when you change batteries often. A crack in the casing is not immediately life threatening because the lamp itself will continue working. But it will then become less water proof and should be replaced on the long run.
I will sporadically update this post when I try out other gear or encounter new experiences.