How long is a simple stiff neck supposed to last?

There no simple. Stiff neck. If you talking aout minor injury it should not last more than few days. If the stiff neck have other cause you have to address that cause for it to go away.Thank you.
Days. Try filling a tube sock with rice, heat it in the microwave to where it is not so hot it will burn you and leave it on your neck for about 20 minutes several times a day. Avoid the motion that gave you the problem.