Typically, check the pressure psi after you first backwash and then once the pressure builds up to 10 to 15 psi higher, backwash again. Or another indication is the return flow back into the pool, if the pressure from the return flow going back into the pool is slow, backwash!

If the pressure is too high, makes sure the pressure gauge reads zero when the pump is off. If it does not fall back down to zero, you will need to replace the pressure gauge.