The Answer:

It's hard to give a firm answer to that question, in part because it's hard to make a firm distinction between the two categories. Smokers and non-smokers aren't locked into one role for life. Where do we place somebody who smoked a pack a day for ten years, and then quit? Or somebody who smokes one cigarette a month? Possibly we can compare lifelong non-smokers with people who have smoked daily throughout their adulthood, but it's easier to compute the amount of expected life lost by people due to smoking in a given year.

According to the Center for Disease Control, in an average year from 1997 to 2001, an estimated 438,000 Americans died prematurely due to smoking. (This includes secondhand smoke, and infant deaths related to smoking during pregnancy.) In fact, 1 in 5 U.S. deaths is smoking-related. Between all these premature deaths in an average year, Americans lost 5.5 million years of expected life, which works out to more than 12 years per person. This doesn't quite answer your question, as it includes some non-smokers and excludes smokers who don't die prematurely, but it definitely drives home the point that smoking is very bad for your health.

Elsewhere on the site, the CDC mentions in passing that, on average, the life expectancy for smokers is 14 years less than for non-smokers, but I don't know how they define those terms.

Fortunately, smoking is on the decline in the U.S. In 2004, 20.9% percent of the adult population smoked, down from 42.4% in 1965.

—The Editors