Natalie (my second) was a very efficient eater from the beginning. Unlike her older sister, whose feeds took 40 minutes or more till 3 months, Natalie managed to gain tons of weight on 10-15-minute feeds. It's only now that she's bigger that she needs longer at the breast.
I agree: if the weight gain is good, and there are sufficient wet and poopy diapers, then you're OK. You can always offer more, but hey, if she's getting enough that she can sleep at night (and so can you) and still gaining OK, then rest easy!
And not to worry, there's bound to be a growth spurt soon that'll make you wonder if she's eating too MUCH!!!! :lol: