Nicotine is the chemical in tobacco that causes most of its effects on the brain. Nicotine is a very addictive drug. It causes changes in a person's mood, blood circulation, and heart rate. Nicotine gets to a person's brain very quickly. I checked out the American Lung Association's web site: American Lung Association to get some information about how long the effects of tobacco use last. According to the ALA, after 20 minutes a person's blood pressure and heart rate return to normal, but it takes 8 hours before the levels of carbon monoxide (a poison) and oxygen in a person's blood get back to normal. Their information is based on smoking.

Chewing tobacco ("Spit" or "dip" tobacco) delivers a lot more nicotine, so I'd guess its effects last longer. The levels of carbon monoxide and oxygen would not be affected by chewing tobacco.

Even after the tobacco or nicotine leaves the body, its effects don't all disappear. Smoking even once a week can cause breathing problems. Smokeless tobacco can cause mouth sores that don't heal. Tobacco use causes bad breath and leaves long-lasting stains on teeth. And of course the tobacco can cause cancer of the mouth, throat, and lungs. There are other health problems as well, such as high blood pressure, stroke, and heart attack.

The good news is that as soon as a person stops using tobacco, a lot of these problems start to get better (see the ALA site for more on this). Quitting won't heal the cancer, unfortunately, but it lowers the risk of new cancer starting. The US Centers for Disease Control has some great information about tobacco for young people at this site:

According to all of the sites I looked at, if you can stay away from tobacco before you turn 18, you will probably never develop this expensive, unattractive, and unhealthy habit.