In general, most air filter manufacturers and HVAC companies recommend changing the air filter every 90 days or 3 months. This may vary depending on the location of your home, if you have pets, and the age of your system and equipment. Older systems often use fiber air filters, which should be replaced every 30 days. Newer systems typically use pleated filters, which should be replaced every 45 days for maximum efficiency.
The most important factor is how often the system is running. The filter collects dust and dirt only when the system is operating, so the more it works, the faster the filter will pick up dirt. During an extremely cold winter in Minnesota, an oven filter will need to be changed more frequently than during a mild Missouri winter. On the other hand, you'll need to change your filter more often during a hot, humid Missouri summer than during the Minnesota summer.
Remember that a filter also gets dirty during AC mode. If you have pets, their hair fibers are large compared to the dust and dirt normally trapped by the filter, so they clog the filter and block the air flow quickly. The more important clean air is, the more often the filter must be cleaned or changed. A coarse-fluid filter, MERV 11 or higher, or an electronic air purifier is best for those with asthma, severe allergies, or respiratory problems.
Cheap fiberglass filters need to be changed less frequently than pleated filters. Coarse-media filters trap more and smaller dust particles, so they obviously clog up faster. Fine fiberglass filters don't trap as much dirt and debris, so they don't clog as quickly. Some of these filters can be cleaned with a nylon brush and placed back in the oven or air controller instead of replacing them.
Cleaning should be done outside or in a garage to keep dust and dirt out of your home. If you clean dirt with a brush instead of replacing the fluid filter, be sure to change it after two or three cleanings. In larger homes, more air flows through the filter than in smaller houses. Because air transports dust, pet hair, and other debris, the filter will get dirty more quickly in a system that works in a large house.
Some people use continuous fan mode to help balance temperatures in their home or draw cold air from their basement to help cool their upper floor. This consumes electricity and increases humidity levels in their home, so it's not recommended as standard operating procedure. The best way to know when to change your air filter is to check it regularly and listen for any wheezing noises when it's clean. If not enough air comes in, there won't be enough hot or cooled air outlet.
These hazards can be easily avoided by checking the filter regularly and changing it when necessary.