If you've ever wondered how much time you'd have were a vampire to find your carotid artery and begin to guzzle, students at the University of Leicester have worked up an analysis. Using fluid dynamics and a few interesting assumptions, they arrived at an average time of 6.4 minutes. Reporting in the Journal of Physics Special Topics, the school's peer-reviewed student journal, the students say that's how long it would take for an average vampire to drain an average human of 15% of his or her blood—any more and your heart rate changes, making blood drinking a "process of diminishing returns," as Gizmodo points out.
The students are assuming that vampires aren't out to kill but rather drink what they can and run (hence draining just 15% of the average human's 5 liters of blood); that each of the five arteries carrying blood away from the aorta are of equal size and smoothness (to establish blood flow velocity); that vampires aren't actually sucking out the blood (thus allowing the students to rely simply on the rate at which blood exits a puncture wound in one's neck); and that vampire fangs leave puncture holes roughly 0.5mm each. So there you have it—on the 85th anniversary of Dracula, we now know that two tiny punctures in a neck will drain 15% of a person's blood in six minutes, 24 seconds. You can now go and write a super-realistic script. (See why an archaeologist calls this a "vampire grave.")