How long do I have to strangle someone to make them just unconscious,

Just look up "how to execute a sleeper hold."

>not killing them or causing brain damage
that's a fine line, friend
focus on arm placement. get their throat right in the pocket of your elbow to constrict blood flow to the brain
when they stop struggling, let them go or they will die.
ask a professional if you want specifics

Go ask /k/, they love this kind of shit over there.