For mild to moderate sinusitis or otitis media in adults, doctors prescribe 500 mg of amoxicillin every 12 hours or 250 mg three times a day, states RxList.com. Severe infections require 875 mg of amoxicillin every 12 hours or 500 mg three times a day. Children need 25 mg/kg/day in two divided doses or 20 mg/kg/day given in three equal doses over a 24-hour period for mild to moderate infection.