How many pack years does it usually take for a smoker to develop smoking related cancer? i know everyone is different. im asking for an average

No clear answer: It is not feasible to provide a meaningful figure. You may wish to use the calculator at this site for your risk. http://www.smoking-facts-and-fiction.com/pack_years.html

Pack-Years Fears: It was once taught that 10 pack-years was a fair predictor of the grim reaper and further studies have shown that even "light smoking" is pretty bad.For example just 1-4 cigarettes a day increases risk three times more than a non-smoker. The British Medical Journal in 2012 (107(3): 406–407. ) stated that "Lung cancer risk is not proportional to pack-years." So it's more complicated than one number