Manufacturers recommend backwashing or cleaning the filter when pressure reaches 10psi above normal operating pressure. Normal operating pressure is what is shown on the gauge after a cleaning or the starting pressure after new filter has been installed. I recommend backwashing a D.E. filter a minimum of once a month and cleaning a cartridge filter a minimum of twice a year (quarterly at best). For sand filters I recommend they are backwashed weekly. However a sand filter is more efficent once it gets a bit dirty so in some cases it is best to wait to backwash you sand filter until the pressure gauge is 10psi above normal operating pressure.