Some people swap them out monthly when being used and about every 6 months if not being used. Running the fans in an open recirculating mode will still cause the filters to "load" with dust and debris. Whether the AC is adding cooling or not. A lot of homes vary in the levels of "dust" that are prevent based on traffic patterns, pets, cleaning practices etc. The real test is to inspect and swap them out based on your particular case. When I worked in the Pharmaceutical industry we had ...»