very true, neglect is a reason why alot of computer components don't last... simply blowing out videocards with an air compressor doesn't always get the dirt buildup. i've had to take apart every videocard i've owned with a bigger heatsink to get all the dust out.
usually every 2 or 3 months is a good idea. dust buildup can make your cards overheat or even burn up in several cases like my friend who burned up an ati x1950xt and an 8800gtx...