How long should my 3 month old (exclusively breast fed) go without pooping before it's a cause for concern?

Ask Dr. if 3-4 days. If a baby is eagerly and happily breastfeeding each day, if he has no tummy aches nor bloating, and if his poops are quite soft, then it is ok for him to poop only once every few days. He is digesting all the breastmilk, and using almost all of it to maintain and grow his body (leaving very little solid material to make poop). The water in the milk is absorbed and later urinated out as pee.