I guess it really depends on how much stress you put them through. Some people type a lot more.
I've killed quite a few keyboards in my time. When I was using rubber domes on my desktops, I would go through about a keyboard a year. I have a mechanical with alps that I have from my 386 that still works. the Keyboard that was with my AT&T 75mhz Pentium died years ago (rubber dome). The gateway keyboard that came with my 450mhz PII died years ago, as did the logitech and memorex keyboards I had afterwards. After that last one died (~'02), I picked up an NMB at a yard sale for a buck and still have it. About a year later, I got another machine and got a model M for it, which lasted me until '09 when the spacebar's spring retainer clip broke. Replaced that with another model M and it lasted me until recently, when it appears someone spilled something on it. Right now at work I am on a rubber dome Dell L100, and I've been using it for about 5 months (and who knows how long it was in service before I came across it), and I'm starting to experience some glitchiness (I have Yakuake mapped to F12, and usually have a browser window open with some utilities, and I was hitting F12 and F11 was activating as well yesterday)
On my laptops, my Thinkpad 380XD is still going strong from '98, but the Sony Vaio, Fujitsu Lifepad, Lenovo Ideapad, and HP Probook 4530s I've bought in the meanwhile have all had key issues. The Probook is a year and a half old, and needs a replacement keyboard because of some flakey keys. The S10e I had for 2 years, and replaced the keyboard after a year, and was going out again after another year when I retired it. the Lifebook I had for about 2 years when I had keys died. The Sony I had for a year or so when it gave up the ghost (it was a lemon from the start).
Both of of those model M's are technically salvagable, I just haven't had the time (full time job as a Linux SysAdmin, plus full time student trying to finish my degree, plus trying to balance a family in all that). It seems like the only projects I've been able to have time for the last few years has been something that helps make things easier for school or work.
For me, since I am a Linux user who spends a LOT of time in the commandline, plus have tons of school papers I have to write (~100 pages worth, this month),