A brand new filter should have a pressure drop of between 2 to 3″ w.c. (water column). Once the pressure drop exceeds 10 to 15″ w.c., the filter should be replaced or changed.
Paper media filter elements are often considered disposable, but can be cleaned. Felt, polyester and wire media can also be cleaned with much longer lifespans. Click here to view filter cleaning instructions.
A simple differential pressure gauge can be screwed into the top of many filter housings. They visually indicate when the filter is in need of maintenance or replacement.