I guess it depends how often hes eating it. My son ate small amounts for a while but he would vomit it all up because he had pyloric stenosis. At five months he was eating 4 ounces ever couple hours. Now hes eight months and has 6 ounces ever few hours, but he also eats baby food, and table food a few times a day and drinks water in a tippy at meal time. So the bottle stretch can be a little longer in between.
I read somewhere it should be about an oz for every hour. So if they go two hrs between feedings they should take 2 oz. 4 if it was 4 hrs. This is what I go by when trying to pump enough for when I am away.