Abour how much does a 2 month old baby feed

I start work tomorrow and em expressing milk for my baby. But I have no idea how much she needs for each feeding. By the way...she is already 15 pounds. She only ever feeds 5-10 minutes on only one boob. WHen pumping, I got 3 ounces out of each boob (6 ounces total) for pumping for 7 minutes. So should I assume maybe she is eating somewhere between 3-5 ounces per feeding. I would think she would be more efficient then a pump at getting milk out????