The quick answer
Clean a filter when pressure rises meaningfully above its clean baseline, flow declines or manufacturer guidance calls for it.
Record clean pressure
After proper cleaning with normal water level and valves, record the gauge reading. This is more useful than a generic number.
Consider debris load
Pollen, dust, leaves, algae recovery and construction shorten the interval. Storms can cause an unusually fast rise.
Procedures differ
Cartridges are removed and rinsed, DE systems require grid and media care, and sand filters use backwashing plus periodic deeper maintenance.
Do not ignore low pressure
Low pressure may indicate restricted suction, low water, a blocked basket or pump trouble. Cleaning is not always the answer.
Inspect while cleaning
Look for torn fabric, cracked manifolds, brittle elements, damaged seals and an unreliable gauge.