Maybe a couple months late now, but if your still looking...
I change them out, along with any other parts depending on my needs and the situation. Try not to base in on an arbitrary measurement of time.
I got 8 years out of my last video cards at least, got about the same from the cards before that.
The video card is one of the few parts that CAN a huge obvious impact on performance, but it will be depend on the other parts as well as how optimized and clean the system itself is. Buying new hardware for a poorly set-up system will rarely solve major issues caused software or other hardware.
Your also gonna want to be sure that your system can handle the new card you wanna install, no point in dropping $600+ on a video card that your motherboard or CPU is gonna bottleneck.
I always recommend tweakguides.com as a great resource for some solid basic hardware advice, and great guides for optimizing the operating system.