I'd got with your gut. Motherly gut instinct is strong and if your instinct is telling you that your baby is still hungry and needs more milk then you're probably right.
I recall at that age feeding for about 15-20 mins on one side, then swapping to the other side if DD wanted more (which was about half the time). I also had massive boobs (14F). I could tell when my boob was empty as it looked and felt empty.