Not as often as you think.

Wait until the pressure gauge rises 8–10 PSI above clean pressure. That’s it.

Most pool owners backwash on a schedule (once a week, after every rain, etc.) and end up wasting water and blasting out good filter media. The filter works best when it’s slightly dirty — the dirt helps catch smaller particles. If you backwash too often, you never build up that fine filtration layer.

Check your pressure gauge regularly. Clean it when it’s dirty. That’s the whole rule. If you don’t have a gauge, install one. They’re cheap and they turn guessing into knowing.

The only exception is if you’ve had an algae bloom or a big debris dump (like a storm). Then backwash when the pressure tells you, then again once the water clears to reset the media. But for normal use? Leave it alone.