Usually chiefly depends on how hot it gets in use.  All solid state electronics wear very slowly in use.  But what really causes significant wear is heat.

Turning equipment on and off causes stresses too.  But again mainly because the of the expansion and contraction caused by heating and cooling.

If you don't overclock a card and your case is properly ventilated and you keep the fans well maintained then a card might last anything from two to twenty years I'd guess.  But then I've never had a card fail so this is entirely guesswork.  Solid state electronics are pretty tough.  Just leave a 3D screensaver running and come back in a century or two and see! 

Thanks for the answer 

I`ll put some fans on my Voodoos right now 

Bye!