In general, you should change your filters once every 30 to 90 days. That said, you may want to change your filters more or less often depending on a few factors. For example, if you are in a highly polluted area, or an area that has a lot of wind and airborne dirt, you’ll have to swap out your air filter more often. You should also replace your filter more often if you have animals in the house (whose dander and hair can clog the air filter), or if you have lots of activity in the house. Likewise, you should replace your filter more often if you or any other residents in the house have allergic sensitivities. On the other hand, if you don’t spend much time at home (perhaps it is a vacation home), or you don’t turn on your furnace all that often, then you won’t have to replace the filter at those intervals. You can always check the box for the manufacturer’s recommended air filter change interval. It’s also fairly easy to inspect your filter to see if it is gunked up. If you can locate your filter near your heating unit, you should be able to pull it out and see if it is covered with dust and debris.