I know theres no age and no scientific answer but average how many years of smoking 15 smokes a day can give lung cancer im asking average not exacly?

10-20 years. To more than double your risk.
Each cig is like. A bullet in your game of russian roulette. It fires damage at your dna, and it takes a successful hit multple years to begin a tumor. I suspect you are in your 20's, and think you can stop, but a pack and a half every 2 days is an addicitve dose of nicotine, and i'll bet you cannot stop when you want like you think you can. Continued use at this rate until age 35 will kill you 50%. Stop f you can.