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 per cent 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 per cent of the body's blood to make it out through a couple of holes in the neck.
To cut down on the variables for their analysis, the Leicester students made a few assumptions. First, vampires aren't out to kill humans. They just want to drink a few life-sustaining sips and leave before anyone shows up with garlic and a crucifix. They also assumed that gravity was negligible (compared to the pressure put on the blood by the circulatory system), and that the vampire wouldn't suck out the blood, ensuring that the pressure on the blood outside the body was no different from that of any other wound. Finally, they assumed that each of the five arteries coming out of the aorta were equally sized and smooth.
By calculating the speed at which the blood moves through the arteries, the students found that a vampire leaving two 0.5mm puncture holes would drain 15 per cent of the body's blood in 6.4 minutes. So if you're writing a vampire novel, making a hyper-realistic vampire movie, or just superstitious, take note and budget your time accordingly.