Give them enough to last them 20 minutes in the morning and the same in the evening. That is all they need.
Yeah? That doesn't seem like much. I guess I could run out and put food in when I get up, then take it out 20 minutes later, then do the same when I get home at night. I could also just feed them before I leave in the morning and pick it up when I get home, 11 hours later, letting them eat all day. That's still less than they eat now, but they'd have an empty crop at night.