As it turns out, vampires need to finish feeding in under seven minutes. Some physics students at the University of Leicester did the math:

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.

The human body can’t function during major blood loss. After 15 percent of the blood leaves the body, the heart rate changes, and blood drinking, even from the carotid artery, becomes a process of diminishing returns. The Leicester students set out to calculate how long it would take for 15 percent of the body’s blood to make it out through a couple of holes in the neck.