here is a good analogy.
if you buy a car, put it on the highway and floor it, how long will it last? myguess: 20,000 miles?
You might be surprised.
http://www.youtube.com/watch?v=Ov8m8gJNeGAFor those not wanting to watch the boring video, saab put a couple of regular 5 door cars on a test circuit and floored them for 100000 Km. IIRC, they only changed tires and topped oil. This is 25 years ago with cars that frankly, where no good (Ive had one for a short while decades ago. To be fair it had 250.000 Km on it when I got it lol, so perhaps they were not that bad).
Anyway, the car analogy is somewhat flawed, as I think flooring a car non stop is probably less abusive than driving it in the city, doing short distances and lots of cold/warm heat cycles. Although some of that actually may apply to gpu's as well (the heat cycles).
Electromigration is the real killer. The longer you run the card, the hotter you run the card, and particularly the higher you set the voltage, the bigger the chance of a card failing due to electro migration. I cant think of a good car analogy, other perhaps than tires wearing out and eventually blowing, but more info here:
en.wikipedia.org/wiki/Electromigrationedit: btw the last paragraph of that wiki article also explain why "baking" a card in an oven can sometimes fix it, if the electromigration occured in a solder joint. Heating it and melting the solder again, can actually fix it.