Tobacco smoke. We all slowly lose lung function as we grow older, typically most non-smokers will die with greater than 80% of their lung function. Smokers lose lung function at a faster rate. If they stop, the rate of decline typically slows to the same rate as non-smokers. Most people do not experience activity limitations until they have less than 80% capacity.
For how long does it take for the cardiovascular system to get back to normal after second-hand smoke exposure?
Moments. Second-hand smoke is a cumulative carcinogen for the lungs, and can trigger asthma which kills people, usually children. I'm less persuaded that it's a factor for atherosclerosis. Know your rights and avoid second-hand smoke.
Depends. Susceptibility to the harmful effects of tobacco varies greatly among people. People with a genetic deficiency in an enzyme inhibitor called alpha 1 antitrypsin may develop emphysema at an early age yet others may never have any noticeable problem at all. Since there is no upside to tobacco exposure, avoidance is the only logical answer.