How much breast milk does a baby need
If your breast feeding your baby you'll feed at least 8 - 12 times a day for the fist month, breast milk digests easier than formula, which means it moves through your baby's digestive system faster and, therefore, makes your baby hungry more often. From the second month to 6th month you baby will feed about 7- 9 times a day.
Before your milk supply is established, breastfeeding should be "on demand" (when your baby is hungry), which is generally every 1½ to 3 hours. As newborns get older, they'll need to nurse less frequently, and may develop a more reliable schedule. Some might feed every hour and a half, whereas others might go 2 or 3 hours between feedings. Newborns should not go more than about 4 hours without feeding, even overnight
We can use this information to estimate the average amount of expressed breast milk baby will need at a feeding:
Estimate the number of times that baby nurses per day (24 hours).
Then divide 25 oz by the number of nursings.
This gives you a “ballpark” figure for the amount of expressed milk your exclusively breastfed baby will need at one feeding.
Example: If baby usually nurses around 8 times per day, you can guess that baby might need around 3 ounces per feeding when mom is away. (25/8=3.1).
Here's a link that might help. Try talking to your pediatrician or lactation consultant for more information.