how old is he...my juvies are 3-4 inches and about 8months old so they get feed once a day as much as they will eat in about 10min if i find any thing left over i remove it cause turts make their tank dirty to begin with i cant imagine extra food lying around in the tank... if they are fullgrown adults you can feed two or three times a week if you want...it really depends on the type of turt...which i forgot what type you have (sonora mud?)
if its sonora i believe that they are mostly carnivores feeding off insects, frogs, snails, other freshwater invertabrae , and on occasion some plant material may be accepted...i would feed two times a week if its an adult