Summarize this article:

Once your waste hose is properly positioned, it is time to turn the pump back on (remember to turn on the circuit breaker as well). Allow the pump to run on the backwash setting for at least two minutes. Although two minutes is a general guideline, the best way to determine whether or not you have adequately back washed your sand filter is by keeping an eye on the sight glass. When the water flowing through the sight glass runs clear, you know that your filter is clean. After you have completed your backwash cycle, you will need to change to a new setting. Once again, before changing the settings on your valve, you must turn the filter completely off.

Summary:
Run the pump for two minutes. Watch the sight glass. Turn the pump back off.