A Jan 3, 2006 article by Nick Schirripa about the difficulty of quitting cigarettes (for more, see ACSH's Kicking Butts in the Twenty-First Century) notes a statistic on quit rates gleaned from ACSH:
For many smokers, the arrival of the new year means one thing: It's time to quit smoking.
About 40% of smokers try to kick the habit each year, and it's no easy task.
According to the American Council on Science and Health, only about 5% succeed on their first try, and about half succeed after several attempts.