From what I have read it is believed that most babies are capable of 5hrs by around 8 weeks and 12hrs by 4-6 months. Whether they actually do this though depends on a variety of factors but I think it is like 80% of babies are sleeping through by 9 months or 70%? Around that.

Personally, I think one of the biggest things that influence it is natural metabolism. My DD1 has a fast metabolism so is skinny, small, constantly snacking and eating, small weight gains; was the same when she was breastfeeding and didn't sleep through till almost 3 and I really believe that she was just hungry, she still wakes up in the morning famished (has toast, cereal and fruit for breakfast lol). DD2's metabolism is slower so she is chunkier, holds more fat, gains weight quickly and does not eat nearly as frequently so is already going long stretches overnight and did from quite early (couple of months). Both were fed the same, slept in the same space etc so it feels like that is what differentiates between their completely different sleep habits. What do you think your DD's metabolism is like? How frequently are day feeds? How much weight gain? Does she hold much fat? That is where I would be looking for an answer.