1.
Ducky 1008XM; no mods.
Used continuously during its life.
Time before minor problems: 0; case wobble and ping out of the box. A few days for keycap lettering to start to wear off.
Time before major problems: A little bit before two months, some keys started to drop keystrokes.
Time before death: Two months, after which backspace ceased to function entirely. Opening the keyboard revealed that the traces had started to pull free of the PCB, leading to loss of functionality.
2.
Noppoo TANK; no mods.
Used continuously for 1 year, then occasionally for another year.
Time before minor problems: After a little bit over a year, some chatter began to develop, especially on the "i" key. Unfortunately, unlike many Noppoo products, the TANK model does not feature tremble control.
Time before major problems: After around a year and a half, the chatter on "i" worsened to the point where it started to be rather annoying.
Time before death: After two years, the chatter got to the point where I no longer bothered to use the keyboard.
3.
Model M; no mods.
Bought used, and has been used only rarely since then. Have owned for ~2.5 years.
Time before minor problems: Keyboard arrived unacceptably dirty. Cleaning solved this issue, but also resulted in a dead Num Lock LED, a non-clicky F9 key, decreased clickiness on backspace, and increased general inconsistency.
Time before major problems: After around 2 years, the keyboard developed a problem of sometimes going completely dead (temporarily) for no reason, other times losing the functionality of some keys (temporarily) for no reason, and other times responding very, very slowly (i.e. character takes a second or two before it gets sent to the system).
Time before death: It's still useable...barely. But the problems are sufficiently annoying that I don't take it out much.
4.
MicroConnectors D07-135TNG; no mods.
Had it for a few months, during which it was used continuously, before disassembling it.
Time before minor problems: After a couple of months, one of the little stick-on anti-skid feet fell off. It probably could have just been pressed back on, but I couldn't find it.
Time before major problems: N/A.
Time before death: Well, it was a Mac keyboard, and I switched to Windows, and at the same time got the (much fancier) Noppoo, so I no longer needed the D07-135. So I took it apart in the hopes of making something out of its parts and the remnants of the Ducky. The attempt was unsuccessful due to the severity of the existing damage to the Ducky, and the fact that I suck at soldering and at making things in general.
5.
I-Rocks KR-6260
Couldn't have been more than a few months, during which it was used continuously.
Time before minor problems: N/A
Time before major problems: There was something kinda...not right about this keyboard. It loved to drop certain characters during fast typing, no matter how hard the keys were pressed...for instance, "keyboard" almost always came out "keyboad." And what good is a keyboard that you can't even write things about keyboards with?
Time before death: So when I got the 1008XM, a "real" mechanical, I figured I no longer needed the I-Rocks anymore. So I tried to convert it to spring over dome for superior keyfeel, using bits of a random non-functional old keyboard that I found in a thrift shop. Naturally, the attempt failed miserably, due to my aforementioned suckiness at making things (though I should point out that this was not an impossible project...the spring-over-dome modules were actually sized such that someone with tools and skills (i.e. not me) could have spliced them into the slider plate of the I-Rocks. Of course, it probably still wouldn't have been able to type the word "keyboard", or many other words, because the mod wouldn't change any of the electronic components.)
6.
Das Keyboard Ultimate; no mods.
Bought used at a thrift shop, clean, but with some cosmetic damage to the case. I've had it for around a year and a half now, during which it has been used continuously.
Time before minor problems: N/A.
Time before major problems: N/A.
Time before death: N/A; still going strong.
7.
IBM KUM6323
Bought used, have owned for almost a few years, but rarely used. Basically, this is the backup keyboard for when all else fails; its longest period of use was for a few weeks after the Ducky died and before I got the D07-135TNG.
Time before minor problems: Well, it keeps increasing in stiff-mushiness as it ages, to the point where it's slow and uncomfortable to use. But of course, this is to be expected with an old dome, even if it's heavier-built than many mechanicals.
Time before major problems: N/A.
Time before death: N/A. Though, at this point, it's really not fit for use outside of desperate circumstances...