How long does marijuana stay in a hair folicle test?

Forever. Sort of... The drug goes into the hair and stays in the hair that grew during that duration. So if the mj was used in in june and the hair grew 1/2 inch in june, it would be in that 1/2 inch, and not in the 1/2 inch that grew in july. So when you test the hair you know that hair close to the follicle represents recent use.