How often your filters should be replaced or changed is largely dependent on the type of furnace filter currently in place. Although changing your furnace filter should be completed every month to every year. Furthermore, the recommended replacement frequency for filters can be found within the product information that’s included when you purchase a new air filter as provided by the manufacturers. However, to give you a starting point for what frequency you’re looking for changing your filter – size matters. For instance, for 1- to 2-inch filters, replace them every 1 to 3 months. For 3- to 4-inch filters, replace them every 6 to 9 months. For 5- to 6-inch filters, replace them every 9 to 12 months.