Determining how often to change your AC filter is based on several things. First, the presence of pets. Pets contribute a lot of foreign matter to the air that finds its way into the AC filters. Pets usually require a filter change monthly. Second, the general environment where your home is built. A dusty environment will be evident in the filters of your air conditioner. Typically filters are changed every 3 months but sometimes environmental factors dictate more often. Third, the lung health of a home's occupants. Asthma or allergy sufferers will need higher-rated filters changed more often to keep the air cleaner. This will lessen their symptoms and contribute to greater health.