research tells us that exclusively breastfed babies take in an average of 25 oz (750 mL) per day between the ages of 1 month and 6 months. Different babies take in different amounts of milk; a typical range of milk intakes is 19-30 oz per day (570-900 mL per day).

We can use this information to estimate the average amount of milk baby will need at a feeding if uusing expressed milk:

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).

http://www.kellymom.com/bf/pumping/milkstorage.html#storage

Thank you for that, very useful! I made it a sticky for you!