2 to 3 years before your risk starts decreasing. I must stress, however, that the risk is progressive and that lung cancer development is proportional to the number of years smoked, the intensity of smoking and smoke exposure, so that decreasing the number of cigarettes per day and the number of years smoked all help. There are no safe cigarettes. The best is a filtered one with reduced tar because tar contains most of the carcinogens. The tar content is where most of the bad carcinogens are found so a very low tar cigarette is better.
Light cigarettes are better than the old ones.
The best thing is just to try to cut down the number you smoke per day and use gum or patch to take care of the craving, the nicotine addiction. Herbal cigarettes that don't contain tars and nicotine are better by far. They replace the mechanics of smoking so people who have the habit or oral gratification of smoking can use those. However, all burned vegetable products have some potential for carcinogenesis. Obviously lung cancer per se is not contagious.