When you quit smoking cigarettes your lungs usually begin the healing process within 1 month as long as you haven't been smoking during that month.
Then it can take 6 months to a year and sometimes as long as 2 or 3 years for your lungs to fully heal up after smoking and you should then see your lung function improving and you should be able to breathe much better.
Quitting smoking is one of the best things you can do to improve your health and help you live a longer and healthier life.
You should also exercise and eat healthy as well but smoking is very bad for your health so you should quit as soon as possible as in right now if you smoke and if you plan to smoke you should never smoke at all.
My Grandpa died at age 60 and he was a very heavy smoker and had smoked cigarettes for many years and he got lung cancer as a result.