It takes about 30 to 120 minutes depending on the location of the sensor, the type of skin, and many other factors. Such a sensor was patented by a Canadian named Dennis Bellehumeur in 2005, but so far the product has not come to market. A company in Colorado called Alcohol Monitoring Systems Inc. makes an ankle bracelet called SCRAM that is currently used by many American courts to monitor alcohol use in repeatedly convicted and violent alcoholics. An excellent review of alcohol skin sensor technology by Patrick Barone, a Michigan defense attorney, has an explanation of how these devices work. It also has a good bibliography of books and scientific research papers on the detection of alcohol levels through skin.