Newborns eat 20-30 oz on average, so you're obviously double that but that is okay. Many here are HVPs (high volume producers) and some of us are considered to be blessed (I say that sarcastically) as having hyperlactation.
Last time after my last surro son, I was pumping at least 100 oz a day at two weeks postpartum. Obviously well above and beyond what a child at two weeks old would eat.
15-18 min at this point doesn't seem like much for each time you pump but if it works for you, that's all that matters. I pump every 2-3 hours for the first few weeks to work on establishing a good supply (and KEEPING IT as your supply can tank so easily the first few weeks) and then pump for 3-5 minutes after the last few drops come out - so that's about 25-30 min each time I pump.
TS x 3 - 03/08, 06/10 & 06/12