It is impossible to say precisely because every baby is different and has different needs. This could be because of size and weight or hereditary reasons. A newborn baby's stomach is only the size of a large walnut so he/she hardly needs anything to feel full. When your milk has come in and you have started breastfeeding your breasts will produce milk on a supply to demand type basis so your baby should be sure to receive as much milk as it needs. After a couple of months of breastfeeding your breasts will learn not to make too much milk in advance and they will stop getting really full and heavy just before a feed. If you are bottle feeding it is easy because a baby will only drink what is needs and won't stop if it is not full. Some guidelines to check that your baby is getting enough to eat is if he/she seems satisfied after a meal, produces five or six wet nappies per day, puts on weight, has soft and moist skin and the inside of the mouth is moist and pink.