How long does it take for a standing person to fall?

This may be a reference to those who faint from a standing position.

While in Army boot camp, we were taught that if placed at a position of attention, we may be required to remain so for lengthy periods of time. That if we locked our knees, this reduced circulation to the lower extremities and resulted in fainting. I actually witnessed this several times in the four years I served. Invariably, someone would lock up, and after a few minutes, pitched forward unconscious. The temperature outside, the heat of the Oklahoma sun, seemed to precipitate a faster reaction, but stress levels seemed to be a factor as well. From my observations, it could take anywhere from five to thirty minutes from the order of execution, to the first soldier falling out, and female soldiers seemed to be more prone to the reaction than males.