What is risk of lung cancer for women over the lifetime if they stopped before 30?

While it not zero... It is remarkably reduced, and closer to the people who never smoked, and life is not a pursuit with zero risk. We try to control the things that we can, and sometimes young folks foolishly start to smoke not realizing the hazard, especially that once hooked it is hard to quit. Also, it's number of years and packs smoked not age stopping.
How much. It really depends on how much smoking was done. Multiply the number of years by the number of packs per day, you come up with a number called pack years. The higher the number the more the risk.
Please clarify. If people stop smoking, their risk for lung cancer is more than non-smokers, but less than if they continued. The risk also decreases the longer a person stays a non-smoker after they stopped.