How long does it take to render an opponent unconcious with a blood choke?

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I have read much about blood chokes. I also have martial arts (Krav Maga) experience, but from reading so many different sources, how long does it take to knock out an opponent with a blood choke applied at maximum pressure. I have read as little as 3 seconds to as much as 15 seconds. For those of you who don't know, a blood choke is a choke that restricts the bloodflow from the cartoid artery to the brain, thus causing a blackout. It can be done many ways, like a sleeper hold, a side-choke, a lapel choke, rear naked choke, etc. Thanks for the answers.