How much milk ??!

My daughter just turned two months old 2 days ago. I'm exclusively feeding her my breast milk. I pump when I want to have a drink or if I'm too exhausted, there's some on hand for my DH to feed her. Now when I feed her from my breast, she spends anywhere from 10-15 minutes on each boob. Now obviously us women don't have transparent breasts, so when you want to feed your baby breastmilk out of a bottle, how do you know how much to give them???