Backwash whenever your filter pressure rises 8-10psi over clean filter pressure. I typically backwash for a minute and a half or two minutes. Watch the discharge water; you can tell when it is getting clearer. Then, put it on rinse for about 30 seconds before back to filter. (Always turn the pump off before turning the multiport valve.)

How long have you had your sand filter? Have you ever added any sand? Sand doesn't need to be replaced but sometimes needs to be topped off if it is getting low.