Here is information from the American Cancer Society with references for each of the benefits and improvements in lung function. it is important to remember that if you or someone who is a chronic smoker quits tobacco use, lung function will improve. However, the degree of improvement may depend on any pre-existing respiratory conditions (e.g. asthma) or conditions that have developed as a result of chronic tobacco use (e.g. chronic obstructive pulmonary disease).

"Can quitting really help a lifelong smoker?"

Yes. It is never too late to quit using tobacco. The sooner smokers quit, the more they can
reduce their chances of getting cancer and other diseases. Within minutes of smoking the
last cigarette, the body begins to restore itself.

20 minutes after quitting
Your heart rate and blood pressure drop.
(Effect of smoking on arterial stiffness and pulse pressure amplification, Mahmud
A, Feely J. 2003. Hypertension:41:183.)

12 hours after quitting
The carbon monoxide level in your blood drops to normal.
(US Surgeon General's Report, 1988, p. 202)

2 weeks to 3 months after quitting
Your circulation improves and your lung function increases.
(US Surgeon General's Report, 1990, pp.193, 194,196, 285, 323)

1 to 9 months after quitting
Coughing and shortness of breath decrease; cilia (tiny hair-like structures that move
mucus out of the lungs) regain normal function in the lungs, increasing the ability to
handle mucus, clean the lungs, and reduce the risk of infection.
(US Surgeon General's Report, 1990, pp. 285-287, 304)

1 year after quitting
The excess risk of coronary heart disease is half that of a smoker's.
(US Surgeon General's Report, 1990, p. vi)

5 years after quitting
Your stroke risk is reduced to that of a non-smoker 5 to 15 years after quitting.
(US Surgeon General's Report, 1990, p. vi)

10 years after quitting
The lung cancer death rate is about half that of a person who is still smoking. The risk of
cancer of the mouth, throat, esophagus, bladder, cervix, and pancreas decreases.
(US Surgeon General's Report, 1990, pp. vi, 131, 148, 152, 155, 164,166)

15 years after quittingThe risk of coronary heart disease is that of a non-smoker's.
(US Surgeon General's Report, 1990, p. vi)

Reference:
American Cancer Society
Questions about Smoking, Tobacco And Health
http://www.cancer.org/acs/groups/cid/documents/webcontent/002974-pdf.pdf