How often should a patient be repositioned if he or she cannot move himself?

It should be done every two to three hours, just as often as you provide care for incontinence. It is VERY important to reposition as this can prevent ulcers /bed sores. There are different ways to reposition you should check google for ways that suit your personal preference and that are appropriate for the patient. I suggest using plenty of pillows or wedges. Hope this helps.