Physically cleaning the outside, I clean my keyboard once a month (its a laptop keyboard, the matte finish has worn off and it gets greasy), I hardly ever clean the mouse off.
As for the inside of a mouse, I suppose, once a month. Sometimes I pick up one at school and clean it out if its really bad... so, numbers for that arent exactly sure.
While attempting to post, I got two: "Someone has replied, please review your post" messages..