How often do I have to get up to prevent pressure ulcers?

Constantly. There is no specific time recommendation but you should shift every few minutes. This applies to any pressure point not just the sacrum or the heels.
Often. While you might want a concrete figure, pressure ulcer formation is not an exact science. Suffice it to say avoiding constant pressure is one of the goals in prevention.
Pressure Ulcer Help. The actual amount of time that it takes to develop a pressure ulcer will vary from individual to individual. There are many factors that would allow a better answer; diabetes mellitus, prior history of ulcers in the same area, moisture from perspiration or incontinence, fungal infections of the skin, low blood pressure are examples. Even moving every 20 minutes sometimes is necessary to prevent.