I'm gonna guess...more than once a year and less than 8 times a day.
It's dust, dude. It's not a science. (Science experiment, maybe...)
If you look in the box and see a little dust on the floor of the box or on a fan, no worries. If you see big dust-bunnies, dust frosted all over the fans and RAM, and the mere act of opening the case causes a cloud of dust to form...clean it.
I'm sorry, but we're just not qualified dust-quantifiers. The only one we had died 12 years ago from gray-lung....