I've always heard that each cigarette takes 15 minutes off your life, but who really knows?
The question is hypothetical. Smoking affects different people in different ways. You may see very old people who have smoked all of their lives and are still smoking at 90+ years of age. On the other hand, a person of 40 may have a fatal disease related to smoking. Therefore the question is as un-answerable as the question asking 'How long is a piece of string?'
about an hour per cigarette.
I am almost 70 and have smoked around 500,000 cigarettes, that would be a low estimate.
I have COPD, I would say the cigarettes have not reduced my life span as much as reduced quality of life. So many things I can no longer do.
Perhaps not the answer you want but will give you a little to think about.