How many oz does your BF baby take from bottle?

Amanda.
We have introduced a bottle to baby girl who is 6 weeks old. She has taken to it pretty well, but she seems to be hungry still after the bottle. She had 3.5 oz today and she still seemed to be hungry after. We pace feed, which takes her a good 10-15 minutes. 
I am going back to work in 6 weeks and would like to have an idea of how much she needs for each bottle so we aren't wasting milk.
I know she had about 3.2 oz a week ago when I went to the LC and she did a weighed feed, so I figured 3-3.5 oz would be enough.