How long does a person have to lie in bed for before getting a bed sore?

8 hours: Of constant pressure causes muscle necrosis. 24 hours to cause skin to die.

4 hours or less: Studies have shown that laying in bed for as little as four hours on a bony area can lead to a full thickness pressure sore to the bone. Things like pressure relief mattresses can buy some extra time but the is no substitute for getting up and moving around.