My laptop... never. I really don't fancy opening it up.
The various desktops around here... whenever the mood strikes. Which is usually when I'm in the middle of upgrading something (like I'm going to be doing here shortly) and I happen to notice any dust buildup.
I do believe, however, that cleaning is only good if you get the dust *out*. If all you do is push it into more electrical stuff, cleaning it can be worse than leaving it alone. So in my mind, it almost becomes a disassembly routine -- take card out, clean it, reseat it, repeat...
