Filters are a vital part of you home's indoor air quality. They are responsible for keeping dust and other allergen causing things from the air you breathe. One should at least inspect the filter every 30 days and change it if necessary. If clean, check at least every 45 days.
If a central air conditioning and heating system is used regularly, the filter should be changed at least once a month.
A filter change costs a couple of dollars, but it is the most important thing you can do to avoid problems later. if you let it go, the blower motor works harder, shortening its life or your evaporator coil could become your filter, blocking airflow and causing it to start icing up; this can cost hundreds of dollars to remedy.