Generally, breast feeding helps drop the baby weight. Usually, nursing moms are told to consume ~500 extra calories/day to help the body make milk.
Since we're each so different metabolically, the ability to drop the baby weight will be very individual. It took me 10 weeks to fit back into pre-pregnancy clothes after baby #1 (exclusive breast feeding and only 17 lbs gained). It took 2.5 weeks after baby #2 (exclusively breast fed followed by cancer diagnosis which caused the weight loss - 20lbs gained during pregnancy + additional 7).
I'd take the extra few weeks in maternity clothes any day personally.
Good luck if you're trying to make a decision on what route to go. Breastfeeding is generally known to help with weight loss because it consumes so many calories, but you have to eat appropriately to produce the right milk for your child as well.