I think it partly depends on how motivated you are to lose the weight.
My sister started going to the gym as soon as she could and her weight dropped right off (she did not breastfeed either).
I breastfed exclusively for 1.5 years and it did not magically melt the weight! Fast forward another year, and I am still 22 lbs from my pre-pregnancy weight. I put on a lot of weight being pregnant, and and my son slept badly until we quit nursing him, and I didn't really feel all that motivated to exercise, so I am now facing a hard uphill battle.
My advice to you is to begin exercise when your body is able, start slowly and ramp it up as you can, but get on a regular exercise schedule like my sister did, and watch what you eat.
I think it may also have to do with how much you gained during pregnancy and also to do with your metabolism. Some of my friends barely show in their 7th month, and they look like the never had a baby two days after delivery. Others are either bigger ladies or put on a ton of weight (like me), and they never really totally have regained their pre-baby state yet.
I still say my main problem is lack of motivation- I am not all that into how I look- so if you are motivated, that should help a lot.
P.S. If you can, do lots of your exercise now while your baby still takes naps and is less mobile. Once they are moving around and sleeping less, your time is sucked into the toddler vortex. and you will have to get a babysitter if you want to exercise in peace.