5 seconds is generally correct. A brain cell can survive for about 5 seconds without oxygen, depending on how active it's metabolism is. The avg. period without breathing before your brain dies is 4-6 minutes because your heart continues pumping for part of that time, and there is still a fair amount of oxygen in your blood without breathing. Even after a few minutes without breathing, there is some oxygen in your blood. As long as there is a small amount of oxygen in your blood, your brain's vital function doesn't end. There are even reports of people (especially children) having 'drowned' in cold water surviving after 45 minutes because the body's and brain's metabolic rate decreased so much in cold water, and they consumed oxygen from the blood much more slowly. In essence, those people never actually drowned because the brain cells never died because the metabolic rate was so slow. But it is true that a brain cell will only survive without oxygen for 5 seconds. That's why cutting off the blood supply to the brain, as in hanging, doesn't take 4-6 minutes to kill someone, but is probably closer to 20 seconds. In that case, no oxygen is getting delivered to the brain because no blood is getting to the brain.