Backwash when the pressure gauge tells you to.

Backwash when your pressure gauge reads 8–10 psi above your clean baseline pressure, not on a set schedule (weekly, monthly, etc.). The only reliable indicator is pressure rise.

Doing it too often wastes water and doesn’t clean any better. Doing it too rarely lets dirty water bypass the sand and flow back into the pool. So note your pressure after a fresh backwash or a new sand fill, then backwash when it climbs 8–10 psi above that. For most residential filters, that’s every 1–3 weeks during heavy use.

As for the sand itself — replace it every 5–7 years, or sooner if you notice poor filtration even after a good backwash. Over time, sand gets rounded, channels form, and it stops catching fine particles. If your pool stays cloudy or your pressure never quite drops back to baseline, the sand is probably done.

Replace with #20 silica sand (or the filter manufacturer’s recommendation) and fill to the correct level (usually about two-thirds of the tank). It’s a messy job, but it costs less than building a new filter.

Your filter will tell you when it needs attention — just listen to the gauge.