I know that healthy babies know when they are hungry but I had to be very wary when taking that advice with my little one. He was jaundiced and, if I wasn't careful, would sometimes sleep up to six hours between feeds as a newborn. That's not cool when we were trying to get the bilirubin out of his system. Basically he was too exhausted to feed properly when he did get woken up so he would fall asleep in the middle of a feed.
My midwife got grumpy at me for listening to advice that said "he will know when he is hungry" because it didn't apply in our situation.
So yeah, babies who are healthy will know very well when they are hungry, how much to eat and so forth, but watch it if your dear child has jaundice or other ailments, as you may need to be waking them every 3 hours for their regular feed even if they'd sleep through it if left to their own devices.