A bite to the neck and a clean getaway—that’s what a vampire needs. A group of physics students from the University of Leicester calculated exactly how long a vampire would need to accomplish those two things: about 6.4 minutes. They published their findings in the university’s Journal of Physics Special Topics.
Not taking into account the anti-coagulant. 😉
Depends on the vampire type. In some movies they just chew the throat open, in others their fangs are hypodermic needles and the blood drained goes directly into the vampires bloodstream. Any of these differences would vastly affect the speed of the blood loss.