As nicotine enters the body, it is distributed quickly through the bloodstream and crosses the bloodbrain barrier reaching the brain within 10–20 seconds after inhalation. The elimination half-life of nicotine in the body is around two hours.
http://en.wikipedia.org/wiki/Nicotine
Technically, nicotine is not significantly addictive, as nicotine administered alone does not produce significant reinforcing properties.[54]However, after coadministration with an MAOI, such as those found in tobacco, nicotine produces significant behavioral sensitization, a measure of addiction potential. This is similar in effect to amphetamine