I think once the person regains consciousness you could zat them again without killing them but it would vary person to person and not something you would want to try multiple times. At times it appeared that members of sg1 grew a tolerance for getting zatted and/or recovered from the effects of a zat if they saw it coming vs getting zatted in the back. Then they would be out until at least the next scene which usually started w/them waking up from the previous zatting.