The average baby needs 2.5 oz of breast milk per pound of body weight per day. So if you have a 10 lb baby, you multiply by 2.5 and get 25 oz per day. Then divide by the number of feedings per day for a baby under six months which is 8 unless the infant is going through a growth spurt. So 25 lbs divided by 8 feedings is 3.12 oz per bottle which you can round up to the nearest 1/4- 1/2 oz.
The next question from moms is when are growth spurts? The first one usually comes between 10 days and 3 weeks, then six weeks, then three months and six months. The are usually indicated by more frequent feeding and lack of satiation, but wet diapers ensure that milk supply is still evident.
Next moms ask, "How many bottles do I leave?" During a non-growth time, leave one bottle for every 3 hours with an extra frozen bottle just in case of spillage or the instigation of a growth spurt,
While you want to stock pile your breast milk prior to going back to work, use mostly fresh breast milk as it will have the current antibodies your baby needs to fight of any infections to which you and your baby might have been exposed. If you need to use up some frozen milk, use it on Monday and then use the milk you pumped Monday on Tuesday, what you pumped on Tuesday on Wednesday, etc. Then on Friday you can freeze again if you want to or save it for Monday if your stock pile is running low.
Organize and label your breast milk. Getting your milk ready for your work week helps things run smoothly. Using bottle bands like the Breast Milk Bandit bottle bands will help you know which milk is fresh to use each day. They will also help ID your milk for the day care worker. You can even write your baby's name on them if you want to.