One cigarette takes about 11 minutes off your life, according to research by the University of Bristol. With a typical pack containing 20 cigarettes, this means that a person who smokes one pack a day loses more than 3.6 hours out of every day. Over a year, that adds up to 1,314 lost hours, or about 55 days.
There really is no good answer to this question. Whenever you hear an answer to a question like this it's purely anecdotal and unscientific. Some smokers will smoke for 50 years and never develop cancer or any adverse effects. Others will smoke for a few years and develop cancer.