You should swap out the filter of your AC unit every 30 to 90 days, as is recommended by the filter manufacturer. Take note, you’ll only have to swap out the air filter while the HVAC system is consistently being used. Also, be wary that some filters can be cleaned, instead of replaced. You may want to swap out your filter more often if you have pets in the house, if you are sensitive to pollen, if there are high amounts of pollutants in the air where you live, or if you suffer from asthma. Air filters are, generally, fairly inexpensive, and it’s worth the investment to improve the energy efficiency of your HVAC system, while improving the quality of your air.