How many years (average) of smoking (half a pack per day) does it take to get lung cancer?

None... ...Even non-smokers get lung cancer. For people having the genetic machinery to get lung cancer (which isn't everybody), smoking increases the chances of lung cancer by an average of 50-fold. The problem is that we don't yet know what genes make smokers susceptible to cancer. It's best not to roll those dice at all. If you want help stopping, tell your doctor. There are many new options to help.
Lung cancer. It is known that smoking causes lung cancer, but no one knows how to determine exactly how much smoking in a particular individual will cause the cancer to develop.
Lag period. There is generally a 20 year lag period between becoming a smoker (>100 lifetime cigarettes) and the development of cancer. The best thing to do is to quit smoking all together, and that lowers your risk of developing lung cancer.
Maybe only 1. Smoking is inherently unsafe and has no benefits. It is the only legally sold product when used as directed can kill you and harm those around you. Once addicted, you cannot stop voluntarily. It is stupid and makes your breath smell. It leads to impotence and yellow teeth and brown fingers. It costs a fortune. Stop rationalizing that there is a "safe" period to smoke. Not so.
0.1. How many drops of rat poison does it take to make you sick? You are considered a smoker if you've had 100 total cigarettes in your life. That's not too many. But even 1 cigarette contains toxins and carcinogens, so why take a chance?
Unknown. The risk of getting lung cancer increases with the amount smoked, but there are other factors beside smoking that increases an individuals risk of getting lung cancer. There is no "safe" amount you can smoke. All smokers are at greater risk of getting lung cancer compared to non smokers.