It takes about 5 to 7 days from the time you that you consumed any drug to grow from the root before it breaks through your skin of your scalp. An average person hair will grow approx 1/2 per month. So for example, if your a man with hair length of 2 inches the drug or drugs will stay in your hair 120 days and then when you get your haircut it will get cut away and the hair will be clean, providing you have not used within the 120 days. Most hair tests will go back 90 days which requires 1 and 1/2 half inches of hair. Men have the luxury to cut it shorter than the 1 1/2 inches. Woman in most case will have no option but to submit the 1 1/2 inch unless they’re willing to shave there head. Keep in mind if you don’t have head hair they will try for arms, armpit, chest, legs, or back hair. They will never ask for pubic hair.