There’s no one-size-fits-all answer. The frequency of filter changes is driven by how much your heating and air conditioning system operates, which is also driven by your individual climate. Start by checking the system’s filters at least once a month. Hold the used filter up to the light and compare it to a clean “spare.” When light is obscured by captured dust and dirt particles, the old filter should be changed. Keep a record for one year and then replace the filter on that basis. At a minimum, it is always a good idea to change filters at the start of the heating and cooling seasons and then in between according to your need. Also, it is a good idea to have your heating and air system checked at the beginning of heating and cooling season to insure proper operation.