72 Hours. In as little as 72 hours after you quit smoking, your lungs will begin to heal themselves. Airway inflammation decreases and the bronchial tubes begin to relax.Breathing becomes slightly easier and lung functionality begins to increase. Weeks to 3 Months. It is between a couple of weeks and a few months that your lung capacity increases significantly; lung capacity can improve by as much as 30 percent. This is most noticeable as walking and other physical exercise becomes easier.