What kind of girl

Joe is having a drink in his local bar when in walks this gorgeous woman. Joe, not being too shy, goes up and sits next to her. He buys her a drink and then another and then another.
After this and the accompanying small talk, Joe asks her back to his place for a "good time."
"Look," says the woman, "what do you think I am? I don't turn into a slut after 3 drinks, you know!"
"OK," replies Joe, "so how many does it take?"