Mine need to be cleaned at least every 6 months, by which time they are completely encrusted by dust.
I clean my AIO fins with a brish i keep for the very reason. And frankly they could easily be cleaned every 3 months and still produce a ton of dust.
Dust not a huge factor here either, which is probably a good thing, since I tend to buy $20-40 cases, which generally don't have dust filters. Maybe some day I'll spend on a "fancy" case, with all of the trimmings.