It should be done on a regular basis
MONTHLY IS A GREAT ROUTINE!
Filters are there for a reason - to trap dirt particles from entering the blower motor and components inside your furnace and keep a consistant clean air flow.
A dirty filter will not only cause your furnace to work harder and less efficiently, but can potentially cause equipment failure.
The need to change the filter will depend on various components of your home and the size of the filter. Dirt, smoke, pets and the number of people living in your house could all contribute to the frequency that the filter may need to be changed. Some filters are disposable, while others are washable or can be vacuumed.
If you want top performance from your heating equipment - check the furnace filter regularly and change it as needed!
NOTE: HVAC Equipment should be maintained regualarly by a qualified contractor. During these visits, components that a homeowner is unable to check, will be cleaned, checked and tested.
CALL AIR-TECH FOR MORE INFORMATION