Most of my 7950 cards have worked since late 2012.
Fans started to break down or make more noise.
My 7990 from 2012 is still running like a champ.
All cards have been overclocked.
OTOH my 7870 from 2013 died.  You can also buy replacement fans on ebay too.  Be prepared to break your screw drivers as some of the screws are cemented in.

I would say expect 3-10 years out of your video cards.