Typically 2-3 times per year, depending on how often you use your heating and cooling system, how dusty your home is, and if you have pets or not. A good rule of thumb is to change 1-2 inch filters every three months, 4 inch filters every six months and 5 inch filters every 12 months. If the filter is visibly dirty it's a good idea to change it. The more dense the dirt, the less efficient the system will be.