It sounds like you have a cartridge filter - might want to add that to your signature. We recommend cleaning pool filters when the pressure rises 25% from the starting pressure of a just-cleaned filter. For example, if your starting pressure with a clean filter is 16 PSI, you would clean when it reaches 20 PSI.
It's time to replace it when the filter shows signs of cracks, punctures, becomes distorted, or cannot be cleaned thoroughly.