The recommendation that most breastfeeding professionals settle on is about 1-1.25 ounces per hour, and ideally not going longer than four hours between feeds. This article gives great information on the ounce per hour rule. It's important to remember that breast milk is different than formula. From the ages of 1-6 month, the total amount baby needs in a 24 hour period really doesn't change (and is about 24-30 ounces per 24 hours). By giving smaller bottles more frequently, it makes it easier for mom to keep up with her supply and baby not getting overfed.