By tobacco I assume you mean Nicotine. Nicotine which is the addictive substance in tobacco smoke will leave your body within 72 hours.
However there are other ways to detect if someone has been smoking, amongst them is the Carbon Monoxide level in their blood and the build up of tar in the lungs, or just lung function in general, these take a lot longer to return to normal, I'd guess 5-10 years.
However there are other ways to detect if someone has been smoking, amongst them is the Carbon Monoxide level in their blood and the build up of tar in the lungs, or just lung function in general, these take a lot longer to return to normal, I'd guess 5-10 years.