How long it would take for a pressure ulcer to form. do they appear rather quickly?

Duration for PUs: Pressure ulcers are due to 'enough' pressure to occlude the blood pressure of the microscopic capillaries. No one is sure how long it you must occlude these capillaries so that a pressure injury results in changes in the skin. It will vary from person to person because of multiple factors in addition to time: repeated episodes, moisture, nutrition, protein stores, ability to heal, diabetes mellitu.

A few hours: Complicated question, but it is generally believed that unrelieved pressure that is high enough to cut off blood flow for two hours can cause a pressure sore. This is why patients who are prone to pressure sores should be repositioned every two hours or placed on a specialized bed that eliminates pressure on the skin.