Eh, it's hard to say. I gained 40 lbs during pregnancy and within 6 weeks I had lost just over 30. I did attribute a lot of that loss to breastfeeding because i was doing very little else. lol. Not exercising and not eating very healthy at all.
But my son is now 15 weeks and i am STUCK. Since about 10 weeks, I've been back at work (at a pretty active job) and on a better diet and i cannot seem to lose anymore. I've heard some women say that as soon as they stop BFing, they drop weight instantly. I'm guessing it has to do with your body holding onto all the excess stored fat to continue making milk... maybe.