I've found myself changing graphics cards every now and then mostly just for personal preference and to "keep with the times". I'm in a Linux environment and sometimes drivers become an issue (legacy), in which case I need to go out and buy a new card as well.
How long does the average graphics card remain useful to the computer as a whole? When should the average user think about changing graphics cards?
You should only ask practical, answerable questions based on actual problems that you face. Avoid asking subjective questions where … every answer is equally valid: “What’s your favorite ______?” your answer is provided along with the question, and you expect more answers: “I use ______ for ______, what do you use?” there is no actual problem to be solved: “I’m curious if other people feel like I do.”- faq – Sathya♦ Jun 30 '11 at 5:48