According to the American Cancer Society, in that amount of time your blood pressure and heart rate drop.
In 12 hours, the carbon monoxide level in your blood is back to normal.
Within weeks or months your circulation and lung function improve.
In a year, you've cut your risk of heart disease in half.
But if you want one more reason for you or a loved one to quit smoking today, on the annual Great American Smokeout, here it is: They have scientific evidence you'll be happier.
The Harvard Study of Human Development, which was actually three studies of Harvard graduates, women, and inner-city men over a six-decade period, ultimately lumped the people into three categories: the happy well, the sad sick and the dead.
What put people into the "happy well" group? Was it education? Income level? A happy childhood?
Nope. As recounted by authors Laurence J. Kotlikoff and Scott Burns in The Coming Generational Storm, the two biggest predictors of being happy and well were not smoking and not abusing alcohol.
Smoking is also one of the biggest influences on the country's life expectancy - which, according to the World Health Organization, is a sad 24th in the world when adjusted to also account for years of bad health.
We're ahead of Cuba - but Canada is ahead of us by a larger margin.
"Whether it is lung cancer, chronic obstructive pulmonary disease, or circulatory and heart problems," write Kotlikoff and Burns, "much of the disability and eventual death that many Americans suffer can be traced directly to smoking."
Kotlikoff and Burns note that historically, the best way to increase life expectancy in America was through public health projects (water, sewer, etc.). "Today, the main levers on health and longevity are directly related to personal choices."
But those personal choices have very public consequences. If the effect of smoking on disability and life expectancy - and even happiness - is anywhere near what they say, there are profound implications for our nation's fiscal future. The country's Medicare and Medicaid programs are on track to be overwhelmed by an aging, health-care-needy population. And our personal choices seem destined to see to it.
What happens if such sources dry up? Won't people more or less be on their own?
Given the overall financial state of the country, it's probably a good idea to take many matters into your own hands anyway. Young people, for instance, may have good reason to believe that Social Security won't be much good to them in their old age. They should be saving for retirement now.
Your health is certainly one thing you can take into your own hands. One of the best things you can do is to stop smoking.
That should be evident enough from all the information above. But consider, too, the amazing story of Patrick Reynolds.
A member of the famous R.J. Reynolds tobacco family, Patrick Reynolds today is a leading anti-smoking crusader - in town today to help University Hospital kick off its tobacco-free campus initiative.
"Someone in our family is on the right side, for a change," he says.
On your side, as it turns out.