The actual electronics of a card will last for several years regardless of performance (I know of old 8086 and 8088 comps that run perfectly well still, just not worth using).
As for how long it will last you with acceptable performance, many people get acceptable performance out of their GF2 MX400 and TNT2 cards, and those are 2-3 years old.
The only wrinkle in that time table is M$'s Longhorn, which could integrade 3D graphics into the desktop, and that could require a lot of power, but no one will be sure till they get close to releasing it.
"Search your feelings you know it to be true, I am your... twin sister" - Darth Vader