A: When they appear dirty. Every house is different. Doors open more with children and pets, so those houses may need a filter change more than someone without children or pets. It also depends on the type of filter you buy.
The tighter it’s woven the more dust it will catch, causing it to clog sooner. We recommend you check your filter once a month until you know how often it should be cleaned or replaced. Keep in mind that it will be dirtier in warmer months due to open windows and doors.