When the time is right and the motivation is strong enough it is possible to quit, however long and heavily you've smoked. Things have to come into line though.
My grandmother quit smoking, cold turkey overnight, having smoked for over 60 years. She was a heavy smoker but we lived with her and when she realised we couldn't go into her room because of the smoke and its effects on us she tossed up which meant more to her - seeing my brother and I or the fags. She chose us. The next day she told us that she had quit and that the windows were open to let the smell out, a month later we all cleaned her room for her (yellow furniture became white!! I will never EVER forget doing that!!) and replaced the curtains etc.
I would say that she never looked back, but that would not be true. Unfortunately it was too late for her and she died of lung cancer anyway. I looked after her and it was one of the most horrible, gut wrenching experiences of my life. Near the end, with tubes in her back draining her lungs, she gasped to us that she wished she could breathe as well as she had been able to when she smoked. I'm actually crying remembering it.
In response to the original question... I think that anyone smoking right by an entrance to a building, or somewhere that forces others to walk through their smoke is being incredibly inconsiderate. My experiences have lead me to have strong feelings on this topic and while I think people have the right to poison themselves, they have no right to force their choices on other people and by positioning themselves where people *have* to walk through their smoke to get somewhere they need to be they are doing just that. There is something in cigarette smoke that affects me personally very badly and if I walk through it it means that I will be coughing and wheezing for days. I used to swim competitively and it would mean I couldn't train at all for several days and not effectively for a week.