The side effects of smoking are complex and varied, and no answer to the question of how fast they depart will do for all issues. Some damage done by smoking can be permanent, causing life-long side effects. Some damage is minor and begins to heal within a day of smoking. People who stop soon after starting may escape any long-term complications. Others who have been smoking longer or more heavily may find themselves burdened with chronic conditions that can't be addressed simply by stopping smoking, though most are vastly improved by quitting.
The Good, Easy Part
If no deep damage has been done to your lungs, heart, and circulatory system, then the side effects of smoking depart very quickly. Within 20 minutes of your last cigarette heart rate and blood pressure begin to go down. Within 24 hours your lungs will have started the slow process of cleaning themselves out. Further, the oxygen level in your blood will rise as you stop contaminating your air supply with large percentages of carbon dioxide. Within two days the actual withdrawal from the drug will be accomplished, though habit and psychological addiction will take longer to overcome.
Over the period of two months to a year the lungs will continue to clear themselves out, improving your breathing ability. Blood pressure will stabilise to a new norm no longer increased by nicotine. Circulation to your hands and feet will improve; skin tone will freshen. During this period the remaining psychological and habitual addictive behaviour will be coming under control and will be less and less demanding. Lungs will regenerate cells and cilia.
By the end of 15 years the benefits of quitting will be in full effect. Risks of cancers will have faded, and the dangers associated with smoking-related diseases will have either passed or improved to the greatest possible degree.
The Tough News
Smoking does damage to the body that cannot always be healed. Once the lungs and bronchial passageways are scarred, the scarring remains. Progressive diseases contracted while smoking can be improved by quitting, but not always cured: For example, the progress of chronic bronchitis and emphysema are slowed but not eliminated.
Damage done to the heart and circulatory system by years of high blood pressure may improve but never fully pass. The risk of cancer after years of smoking never leaves entirely, though the statistical odds drop to near the levels of non-smokers. Further, there is no cure for addiction: A formerly addicted smoker will always be at risk of relapse. The chemical patterns of the brain, once changed by addiction, do not return to their former state after addiction, but remain primed for nicotine.