How Long Does It Take for a Pressure Ulcer to Form?

I recently saw some questions on-line about how long it takes for a pressure ulcer to form, and I was reminded of my own experiences of trying to find an answer to the same question.  I read several medical articles that said that we need to turn people once every couple of hours to prevent pressure ulcers.  I also read other articles that said turning people every 4 hours were fine and yet others that said every 2 hours were not often enough. Why is it that when you get 3 doctors in a room, you get 4 opinions?

Anyways, I decided to find out for myself.  Since pressure ulcers form on or near a bony area, I chose to press my finger against my shin bone for two hours.  So, that's a picture of my left shin (please ignore the dry skin - my foot directionally to the top of the picture) with my right forefinger next to a healing area.  I started off on my experiment with gusto, with a timer on the table and a football game on TV.  My forefinger waged war on the skin of my left shin.  After about 3 changes of possession in the football game (about 15 non-football minutes), I got bored with my experiment, convinced myself that I was not going to prove anything, and quit.  Soon thereafter, I completely forgot about my experiment.

Next day, I am in the shower, soaping my legs... hey, what in the world?...  what is that dark spot on my shin?...  uh, isn't that the spot I was pressing?...  my finger fit perfectly into that dark depression!  I had a pressure ulcer!  What amazes me is that the skin damage that I caused many months ago still hasn't healed completely.  Conclusion to my experiment?  I still don't know how long it takes for pressure ulcers to form, but I now believe that anyone, including a healthy person like myself, can develop pressure ulcers in a short time that will take a long time to heal.