Good question. I know it takes 20 minutes after a hanging to be declared, by way of no heartbeat ... the body wants so very much to live.
But the means and duration can be varied by the executioner's skill and timing. Weight, height, rope type and length, knot placement, rate of trap drop...all of which should be tailored to reflect the horror of their crime. Some get the merciful quick snap and others should dance and gurgle as long as possible while their faces reflect the diversity rainbow of darkening colors.
It's an art and I volunteer my services when they start processing the 2020 usurpers after their trials.