In general, it seems like my controllers have lasted about 4 years each based on reasonable use before I give up on them. The exception seems to be the Gamecube controllers which will probably last forever.
That said, absolutely every controller I have bought is still functional. Every button works as it should, and the analogue sticks are generally the only thing that I get problems with.
Mega Drive - Dpads might be a bit sticky on some of them. Not sure.
N64 - Three analogue sticks worn out by myself in probably 6 years.
Gamecube - Had three controllers since 2003 and all work perfectly.
Dreamcast - Bought in like 2001, loads of controllers, all good!
PS2 - Bought in 2006 and barely played, so both controllers fine.
PS3 - One controller since 2009, used a fair amount, still good.
Wii - Three lots of WiiMote + Nunchuck. Still great.
360 - Two controllers from launch, all stick grips decimated after 8 years of PGR, Halo, CoD, Mass Effect, Gears and the rest...
Bonus Round!
Guitar Hero 2 Xplorer - Good as the day I got it in 2006, and it was hammered for about 200 hours.
Guitar Hero 3 Les Paul - Probably nearing 300 hours of use on Expert. Neck contacts degraded enough to cause frequent note misses, so had to hard-wire it. Fret and strum buttons still feel good, though lacking some of the tactility they used to have. Oh, and I ripped the whammy bar off one day because I got annoyed with it. :\