Nursing should burn calories for as long as your baby is actually nursing.
I think that the number of excess calories needed for nursing is likely greater than the 200-500 estimate mentioned above. Human milk contains about 20 calories per ounce, on average. The rule of thumb for the amount of formula a young infant should drink is 2.5 ounces/ pound. So if a baby weighs 10 pounds, he should drink 25 ounces of formula per day. If a nursing baby drinks the same amount, he’s ingesting about 500 calories. Keep in mind that if an ounce of mother’s milk contains 20 calories, the mother will burn more than 20 calories producing that ounce of milk. And of course, most nursing babies weigh more than 10 pounds. So I think that 500 excess calories is the absolute minimum for most nursing babies who do not receive other food.
I lost all of the pregnancy weight by 3 months postpartum without exercising; I’d guess most of that was due to nursing (nursed exclusively for 6 months). I wear the same size as you.