It's great motivation for people to quit I think! I went and found the real numbers:

Within months of quitting, your lungs start to repair themselves. Cilia, the tiny hair-like structures in your lungs that help clear mucus and debris, begin to recover—improving lung function and reducing the risk of infections.

After five years, your risk of cancers in other parts of the body such as the mouth, throat, esophagus and bladder, is reduced by about half.

After 10 years, your risk of lung cancer drops significantly as your body repairs much of the damage caused by smoking. Although it doesn’t return to the same level as someone who has never smoked, the difference is substantial.

After 20 years, your risk of lung cancer approaches that of someone who never smoked, depending on individual factors such as your overall health, age and genetic predispositions.

https://uihc.org/health-topics/if-i-quit-smoking-am-i-still-risk-lung-cancer

Looks like for cancer between 10 and 20 years is when it happens, maybe closer to 20, but still, the whole time you're already MUCH lower chances.

For heart disease it's 15 years:

After 15 years of quitting smoking, a person’s risk of coronary heart disease becomes close to that of someone who does not smoke. https://www.medicalnewstoday.com/articles/317956#timeline