Running the card at extreme stress (i.e. playing games at max FPS), especially when OC'ing the GPU for extended periods of time (i.e. 18+ hours) while the core temperatures of the GPU is high (greater than 90* C) can stress the card. Gam3r01 is right - the biggest enemy of the computer is going to be dust (it reduces the cooling ability of the cooling fans).
I know of installations where people stress the GPU to the max, never clean it, etc., and it lasts longer than the other components in the computer....and others where perfect attention to cleaning and the GPU died within a year. Some of it is luck....cleaning and keeping temps lower will extend the life of the GPU in most cases.