Depending on where you tie it and how much pressure is placed on certain blood vessels, not long.
11lbs (5kg) of pressure placed on both carotid arteries for will make you pass out.
4.5lbs (2kg) of pressure placed on the jugular for 10 seconds will make you pass out.
33lbs (15kg) of pressure on the trachea is required to completely close it off, so be very careful about that one as it can cause total asphyxiation.
Hope this helps!