As long as your baby seems satified when he is done, that's plenty. They say that a way to tell is by how long they go between feedings. For breastfed babies, the nurses in the hospital told me the average is 2-3 hours and formula fed babies generally go 4-6 hours between feedings. My daughter was breastfed for the first 3 weeks, and even on breast milk she would go about 5 hours before eating. One nurse told me to wake her up and feed her, and another said if she's sleeping to just let her be, but not to let her go more than 6 hours. I followed the second nurses advice, and to this day have not regretted it. So as long as your baby seems satisfied and isn't underweight, don't worry about how much he's eating right now. And yes he will let you know. My daughter did and still does start crying if she feels she's not getting enough in one feeding. I've also noticed that she will fuss (she really isn't a fussy baby) sooner than usual, and as one mom mentioned, eat more when she is going thru a growth spurt.