Three days, if the victim's hunger was full at start, if it wasn't, two days. I've never done this myself, but heard it from a friend, who has tried. If it takes over three days, then I recommend that you'll check if there is any way he gets food, like that witches conjure apples and are hard to be killed this way. If the victim was a vampire or a merman, then you can just take the roof of the room away and sunlight will do it's job.