Breastfed babies (like bottle fed babies), will drink variable amounts of milk at each feed. Individual babies will also vary in how much they prefer to drink, and will need changing amounts of milk as they grow older. How much breast milk you will need to express to leave for your baby, can be a little unpredictable.
It is for this reason that many women will divide their expressed breast milk into smaller 'portions', before freezing for storage. This avoids the need to discard large unused amounts of precious breast milk ('liquid gold') from being poured down the sink, because your baby was not hungry enough to drink it all (you can't refreeze unused milk). If you are unable to supply enough expressed breast milk for the time you will be away from your baby, you may need to make up the rest of their feeds with formula milk.
How much do I need to express?
The following volumes of milk are meant as a guide only. You should feed your baby 'what they need' and not be concerned if they do not finish all the milk in their bottle (or need more at times).
By a week or so after the birth, newborn babies born at term will generally drink about 150mls per kilogram of body weight, per 24 hours. Therefore, if your baby is about 4 kilograms in weight (or about 4,000 grams) they will drink about:
150mls x 4 kg = 600mls (in a 24 hour period)
Babies will normally have about 6 to 8 bottles in a 24 hour period. So, divide 600 mls by 6 feeds, estimating that your baby will drink about 100mls of milk per feed. Be aware that they may drink 80 mils at one feed and 120 mls at the next. If your baby is feeding more frequently than 6 feeds per day, then the amount will probably be less than this. As your baby grows, the volume of milk per bottle will slowly increase (as your baby puts on weight). By the time your baby is about 1 to 2 months old, they will probably be drinking about 120 to 150mls of milk for each bottle. By 3 to 4 months of age, this may increase to about 150 to 220 mls per feed.
At around 4 to 5 months of age, babies will generally decrease their milk requirements. This changes from about 150 mls per kilogram of body weight, to about 120 mls per kilogram of body weight. However, depending on how heavy your baby is, this may still equate to about 150 to 220 mls per bottle because they often need less milk feeds (possibly 4 to 5 bottles in a 24 hour period). While it is accepted that babies can be exclusively fed on breast milk until they are 6 months old, some babies will start on solid foods at around 4 to 5 months, and may also be starting to sleep through at night.
If your baby is premature and/or unwell, they may need to spend some time in the intensive care nursery and may be unable to drink milk feeds for a while (having their nutrition through a drip in their vein). Once they eventually start on milk feeds, the volumes are usually quite small, and subsequently slowly increased over days and weeks, usually on a strict timing regime (1, 2, 3 or 4 hourly). When your baby is ready to come home, they will usually be on about 150mls per kilograms, having about 6 to 8 feeds in 24 hours, and will progress with their feeding schedule for their 'adjusted premature age'. (An example of an 'adjusted premature age' is when a baby who was born 3 months premature is 4 months old, their 'adjusted age' is 1 month old).
Read more about expressing milk for your baby here