Nicotine reaches the brain 10-20 seconds after smoke is inhaled.
"The Health Consequences of Smoking—50 Years of Progress. A Report of the Surgeon General." U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Center for Chronic Disease Prevention and Health Promotion, Office on Smoking and Health. Nicotine, 2014. Report.http://www.surgeongeneral.gov/library/reports/50-years-of-progress/sgr50-chap-5.pdf