I think the most interesting study is Pope et al., 2001, where they compare people who had smoked 5000 times or more, but have basically quit for 3 months, to people who have never smoked, and people who still smoke daily.
The people who have quit for 3 months are the same as the controls.
The question is, how long does it take you to smoke 5000 times. In the paper they say that it is 13 years or so. Which was shown by the costa-rican paper to not be enough to cause damage, becasue in their study, only the peopl who had smoked for 34 years were damaged....
Interesting stuff.