The chemicals in a cigarette and tobacco in general stay in your system for quite a while after you stub the last smoke out. The effects of smoking last even longer, and a few are pretty much irreversible. The good news if you are planning to stop is that some of the benefits appear almost immediately.
Nicotine and carbon monoxide clearing
Nicotine takes about two to four days to leave your system entirely. The physical cravings, as opposed to the psychological ones -- which can start immediately, after weeks or not at all --, may kick in badly at this point. According to the NHS, levels of nicotine and the almost as noxious carbon monoxide halve after about 8 hours and are usually completely or nearly completely gone after 72 hours or less. Like many drugs, traces of nicotine may remain in your hair, which is not alive, until you have a haircut. This, however, has no effect on your health, appearance or cravings.
The effects of tobacco on the body begin to pass on a timeline ranging from 20 minutes to several years after a smoker stops. The NHS publishes a simple timeline demonstrating this. The near immediate effect is that your heart rate, which is accelerated temporarily by smoking tobacco, returns to normal within about 20 minutes. The chemicals start leaving your system within hours. On the other end of the scale, it takes years (and sometimes never) before an ex-smoker is back to the same risk of certain diseases as somebody who has never smoked, depending on individual circumstances.
Some of the other effects of smoking are more subjective. The smoky smell will disappear as soon as you wash yourself and your clothes, although it might linger in your home for longer if you smoked inside. You might notice an improvement in your skin at any time after you stop, probably because the chemicals in tobacco are no longer blocking the uptake of nutrients. For example, according to the University of Maryland, smoking lowers the amount of vitamin C in the body.
Nicotine replacement products
If you are using nicotine patches, gum or electronic cigarettes, the other chemicals in tobacco will be gradually leaving your system once you stop smoking but, obviously, the nicotine will not. Once you have the final piece of chewing gum or take off the last patch, the nicotine will leave your body on the timeline described above. If you have taken these products according to the instructions, you should be on a lower dose than you were as a smoker and should also have dealt with some of the psychological aspects of the habit.
How to speed up the process
Drinking plenty of water may help your body flush out tobacco toxins. It also gives you something to do with your hands and mouth rather than eating or drinking coffee, both of which could have undesirable effects if done to excess. Avoiding alcohol, or at least moderating your intake, while you stop smoking benefits you in two and perhaps three ways. It means you are not taking in another source of toxins, leaving your body free to concentrate on flushing out the tobacco toxins. Because alcohol lowers inhibitions, avoiding too much may also help you resist the temptation of cigarettes. Finally, despite the smoking ban in pubs, people often get into the habit of going outside for a lot of smokes when they drink, leading them to psychologically link the two activities. Avoiding both for the first few weeks of giving up smoking might be sensible.