Depends. I just cleaned my case fans for the first time since they were installed 5.5 years ago, and they only had a light covering of dust (I clean off the filters in front of them every couple of months though). Previous systems I owned with high-speed fans needed cleaning much more often. I clean stuff when it looks dusty, or it starts making funny noises, or gets hotter than it should, or if I happen to be taking it apart anyway.