There is a relatively easy way to figure out how much your baby should eat while you're gone. A full milk supply for a breastfed baby is approximately 30 ounces per 24 hours. If you figure out how many times your baby eats in a 24-hour period, just divide 30 ounces by this number. This is about how much your baby takes per feed. For example: If your baby eats 10 times in 24 hours, then he probably takes about three ounces per feeding. If he only eats eight times per 24 hours, then it would be approximately four ounces per feed.
Once you know about how much your baby typically takes per feeding, then you just need to figure out how many of them you'll be missing during your time apart, and leave enough milk (and a couple of ounces just in case) to cover those feedings.
Another way to look at it: The more times you're able to feed your baby during the time you two are together, the less milk he'll need during the time you're apart. So working in some extra evening and morning feedings is a great way to help reduce some of the stress of "keeping up" while you're at work.