If breastfeeding, your baby will let you know when s/he needs to eat. S/he should eat every 2-3 hours. S/he will stop sucking and refuse the breast when done. If on formula, s/he will eat 2-3 ounces every couple of hours. You know s/he is well fed if s/he has around 7-10 wet diapers and 2-3 bowel movements per day.