don't know, but maybe she takes the bottle after falling asleep as it is easier to drink from the bottle than from the breast!
I offer one boob, and if it is not enough I swap, but this actually only happened in the beginning though, when my milk production was not yet established!
I stop if she either goes off on her own, falls asleep or starts to fuss around on the breast, as this is the indication that sth else is going on (either burp, poo or tiredness)!
after poo or burp I offer more, but not if I can see she fusses around 'cos she is tired! then I try to put her to sleep without breast!
in the beginning I gave her an hour to feed. now it takes her approximately 15 to 30 min average to feed.