My suggestion to every homeowner is to get in the habit of checking your furnace filter monthly. Whether you have a 30 day, 90 day or six month filter, checking your filter monthly can head off potential problems and is a good practice for any proactive homeowner.
Cheap fiberglass filters do very little to improve the air quality in the home. The layer of fiberglass is not nearly thick enough to capture anything but large particles of dust. Many of the most harmful microorganisms are virtually invisible, and require a much more efficient filter in order to be collected. Upgrading the furnace filter to one with a MERV rating above 8 or 9 could drastically improve indoor air quality, but there is one problem. These filters still need to be changed every 2-3 months and the extra cost of the upgrade is simply not practical for many homeowners.
The majority of problems found in air conditioning units are due to lack of maintenance. These units are forced to work coated in dust, dirt, and other debris from the home and the outdoors that obstruct their ability to cool properly. Most often these units fail on particularly hot summer days because of the extra load places on the system. You could prevent these breakdowns easily and affordably by having Four Seasons clean and check the system early in the season.