1 Typical adult doses of cephalexin are 250 mg every 6 hours, or 500 mg every 12 hours. 2  The usual dose for a child is 25 to 50 milligrams (mg) per kilogram of weight, given in divided doses.3  Keflex capsules come in 250, 333, 500, and 750 mg options.ephalexin is prescribed to treat respiratory tract, middle ear, skin, bone, and urinary tract infections (UTI). It's also used to prevent infections caused by streptococcal bacteria, including prevention of rheumatic fever.