This really depends on the type of system and filter that you have and how dirty your home is. In general, disposable filters (the kind you can buy at the big box stores) need to be changed once a month. If you have several shedding pets, they may need to be changed more often. There are filters however, that only need to be changed yearly. Your Air Flow Designs service technician can recommend a filter change schedule specific to your situation during your yearly tuneup.