Smokers benefit almost immediately from quitting. Within 20 minutes, blood pressure and pulse begin to improve. After a day, most of the carbon monoxide will be gone and some will already notice improved shortness of breath. After one year, the heart attack risk drops 50% and similarly after 10 years, the lung cancer risk drops 50%. The risk of stroke normalizes after 5-15 years and the risk of heart attack normalizes after 15 years.
Sources: Smoking Cessation: An Overview. National Center for Tobacco Free Kids; U.S. Department of Health and Human Services. The Health Consequences of Smoking: A Report of the Surgeon General, 2004; American Cancer Society, When Smokers Quit – the Health Benefits of Quitting