It depends on the person and what her "set point," or comfortable weight was before she got pregnant. Many mothers lose weight easily while breastfeeding because lactating burns 500 to 600 calories a day, but many mothers have to add healthy exercise and eat carefully to lose the last bit of baby weight. (Personally, after my first I reduced my saturated fat intake and was as thin as I'd been since high school. After my second, it took a full year to get back to my pre-baby weight and I needed to exercise and avoid cupcakes more diligently! Same with my third, but the bulk of the baby weight peeled off within the first 6-7 months from breastfeeding. Just cookie and cupcake weight left ;)
Consider contacting your local La Leche League Leader for free, medically-accurate information about breastfeeding. LLL has saved my sanity dozens of times! It's such a terrific community service! They usually have books that are free for anyone to borrow. I recommend "Eat Well, Lose Weight, While Breastfeeding." You can find your nearby LLL contact at www.lllusa.org. Meetings are fun and low-key and are a good way to meet other nursing moms with babies the same age as yours.
Best wishes!