The nicotine in cigarettes is an extremely addictive substance. Smokers begin to enter nicotine withdrawal only 20 minutes after finishing a cigarette, and it takes about two and a half cigarettes before the average smoker has a full dose of nicotine [source: Volkow]. Those highly addictive properties explain why most smokers smoke regularly throughout the day, and why some smoke almost without stopping. The short time that nicotine actually affects the body also explains why smokers have such a hard time quitting; it doesn't take very long to crave the next cigarette. The physical addiction to nicotine is usually gone only a few weeks after quitting, however. After quitting, you won't have to spend time worrying about what you'll do if you run out of cigarettes late at night, how you will be able to smoke if you have a long flight or when you can take your next smoke break during a day at work.