That's a pretty long time! Are you sure that she was actively drinking the whole time (her jaw is dropping low and pausing, you might be able to see/hear a swallow, there's a definite suck/swallow/breathe pattern instead of regular breathing throughout)? It seems likely that she's spending time just sucking and not drinking, which could be hard on your nipples in the long run - it's ok to take her off if she's just nibbling. As far as milk being left - it depends on your supply. Baby doesn't necessarily need to empty the breast every time she feeds, but you should at least be able to feel a difference beginning to end of one side. If you can't, I would wonder about how well milk is transferring.