Physically I'd say it depends on your sense of what dirty and clean means and on how often you clean your housing in general. In a dusty home your computer will be more likely to be dusty.
I clean my computer (tower) rarely, maybe every few months.
My mouse rarely because I have relatively clean hands.
My keyboard every once in a while (cotton swabs between keys). Like every 1-3 months
and my monitor depends but not too often as well.>digitaly
Well depends on what youre doing w/ it. And if you expect to potentially be visited by our guarding friends.