I've only fainted twice, but they happened within the space of about 20 seconds. I had a stomach bug and so had eaten hardly anything for a couple of days. I got up in the middle of the night - too quickly, I guess - and fainted in front of the kitchen sink. A few moments later, I regained consciousness, stood up, and promptly fainted again. The second time I woke up, I had the sense to lie down flat on the floor and yell to get somebody's help.