Immediately after birth, one will lose about 12-15 pounds (weight of the baby, placenta, amniotic fluid), however, don't be so excited to weigh yourself right after delivery, because the number may still be high because of extra fluid that you retain in your legs and body, especially after a long labor with lots of IV fluids, and the breasts fill with fluid and milk.
Most patients who are average weight who gain an average amount of weight during pregnancy, come into their 6 week checkup with an extra 5-10 pounds, and most lose all but 3-5 pounds by their 6 month postpartum checkup.