How far does it go back?
Hair testing generally uses 1-1/2 inches of hair, representing about 3 months’ growth (head hair normally grows at about 1/2 inch per month). It is generally accepted that in order to test positive, the drug in question must have been used 3 times or more within the window of the test. After a drug is used, it takes about 7-10 days for the hair containing the drug to grow out of the scalp enough to be cut. Therefore, the hair test will not include drugs used in the week prior to the test. The drug remains in the hair, growing out much the same as hair dye and "dark roots". Eventually that hair containing the drug is cut off with normal haircuts.
-even better, my hair grows hella fast so I cut it once a week...