Your sand filter should be backwashed once the pressure reaches above 20-25 psi. Pressure is different for every pool, filter, pipe size, plumbing arrangement, and heating combination, and depends on your particular combination.
The psi can range from 15-30 before it requires backwashing. Start with a baseline pressure, which is the pressure your filter runs at right after it has been backwashed. Once that pressure increases 7-10 psi, it is best to backwash it for 2 minutes, or until the clear sight glass bulb is clean. Then, rinse for 1 minute.
Backwashing sand filters too frequently is counter-productive, as the accumulated dirt trapped in the sand actually aids in trapping the finer particles in the pool water.