Della recently quit her 20-year smoking habit. how long will it take for her risk of cardiovascular disease to become the same as a person who has never smoked

It will take 15 years for Della to be in the same risk of non-smoking persons in risk of cardiovascular disease. As you quit smoking, the lungs will start to purify itself, but it will take some time. On the other hand, it cannot be assured that the lungs may be totally restored, more or less it would just be clean over a period of time.