Usually every few weeks because we have dogs, and the short-hair one sheds like crazy. I imagine if we didn't have them, it would be months in between cleanings.
bad idea, even in jest, deleted.when I think about it, I should find some solution to the hair shedding.
I usually clean them once a month. With positive air pressure my front panel gets quite dusty, even at low 500RPM fan speeds (3 x 120mm intakes).