You’ll need enough milk to feed your baby for a full day. The exact number of ounces will depend upon how old your baby is and how much he’s drinking.
But, on average, you’ll need about 25 ounces of breast milk in a 24 hour period (source). If you’d like to aim a bit higher just in case your baby is a big eater, 30 ounces should keep you covered.
If you have a baby that’s still night nursing, or even nursing several times before and after work, you would need to pump less milk during the day while you are at work.
Julie Matheney (IBCLC) of LA Lactation shares her experience:
“I night nursed 3-4 times a night until my daughter was 1 and once or twice a night until 18 months since I was working full time and she reverse cycled (wanted to directly breastfeed with me) so I only needed to pump 10-12 ounces for her while I was gone on an eight hour shift and she made up for the rest at night.
That fact can help many moms as I would never expect them to pump 25-30 ounces during the day if they’re night nursing.
Many moms become discouraged and quit breastfeeding early because they think they don’t have an adequate supply when they actually do. I usually tell moms to take 25 and divide it by the number of breastfeedings in a 24 hour period while they’re home on the weekend. That’s an average amount per feeding. Mom can then figure out how many feedings baby gets while she’s gone and just leave what is needed.”