At least 1.5 inches. The hair drug testing standard procedure is to procure a 1.5-inch long hair strand specimen. The different drug tests are then conducted with that hair strand. If you don't have a 1.5-inch long hair on your head, the testing personnel can take a sample from your armpit, legs, and so on. Nonetheless, that hair sample too has to be at least 1.5 inches long.