It stays in the hair forever so long as does alcohol, at least in the sort of industrial quantities that might apply to a dipsomaniac. There is at least one doctor at LGW who has been agitating for hair analysis checks for years.
Two things only prevent this happening as a matter of course. One is the cost and the second is that the CAA are quite concerned at the number of positive drug tests that would be thrown up by such a testing plan. I conjecture that they're worried about press exposure but suggest that since the demise of the NoW much of the danger has been removed.