How much breastmilk in a bottle?

I am returning to work in 3 weeks and am wondering how many ounces of breastmilk per feeding does your baby take? My mom is going to be watching him, and so this morning my 9 week old had 3oz in a bottle (slow nipple and paced feeding) and he still wanted more so I nursed him some. An hour later after playing and being awake, I nursed him again until he fell asleep. I don't want to underfeed him but also don't want to overfeed or waste pumped breastmilk! Also, due to the nature of being military, I will be gone 12-13 hours per day (😭), so I am wondering what is an average amount if milk he will eat during that time? I have read breastfed babies eat around 25-30ozs in a 24 hour period. My plan is to pump at work and use that milk for the next day, but I also have a freezer stash in case he needs more. Thanks!