Depending on how much marijuana you use, the drug can stay in your system for a long time. I heard years ago that the roots of your hair can retain drug use info for well over a month. And that was back when the tests weren’t as advanced as they are now.
hair tests can go longer than a month. if you had the hair while you did the drug, there are traces of it.
marijuana will be present in a bloodstream long after any effects that the person can feel.