I clean them out probably once a month. The replaceable ones are supposed to be changed every 6 months.

We clean ours with three cats when they look bad.. Keep them instock so you have when needed!

Your HEPA should be changed every six months.  Your washable (or non washable if it isn't a Bissell they probably aren't) filters should be cleaned at least once a month.  If you vacuum a lot maybe twice a month to keep your vacuum suction strong.