If you mean after you have personally taken a substance I don't think it's long (hubby not here to ask), however if for instance drugs dogs came into your property where you had taken the drugs they would be able to detect that a substance had been there even if there was none present at the time of searching, if the drugs are hidden even if up in the loft a drugs dog will detect them, remember that they are trained with actual samples of specific drugs so know the scent, it becomes second nature to them, I was in the army working with search dogs, at that time searching for explosives, it is really clever the way it is done.