Typically, check the pressure (psi) on your gauge after you first backwash and then once the pressure goes up 10 to 15 psi, backwash again.  Another indication is the return flow back into the pool...if the pressure 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.