Check your furnace or AC filter monthly and replace it at least every 3 months
Added
Locate the filter on your furnace, air handler, or return grille. Check it once a month; if it looks dirty, replace it (or clean it if it is reusable). At a minimum, put in a fresh filter every three months. Match the size printed on the old filter and face the airflow arrow toward the blower.
The result
Result: U.S. Department of Energy: dirty filters reduce airflow so the system works harder and costs more to run; check monthly and replace at least once every three months.
Source
U.S. Department of Energy (opens the original source in a new tab). Check it for the latest numbers before you act.