I had an A1200 that was on for 99% of the time from 1993 to 2007 (by then in a tower) never had a fault or anything - until I sold it. Probably because it was always on.
The 2000s never seem to die - pretty heavy duty construction. Power supply may slowly die or the fan might make noise, but they're easily replaceable.
I have had my share of used A500s which had various problems. The chips in such machines aren't really pushed to any thermal limits, so other than accelerators or disk controllers, I can't see why they couldn't last for a very long time, whatever very long time means.
