Keegan is crated when I go to work, so she is in there for up to 8-9 hours. She has been crated since she was 6 weeks old. I don't have the luxury of coming home on my lunch to let her out. She gets along fine, now that she is older, she is out at nite, and sleeps with me, and she holds it. There have been times when I was sick or extremely tired and have slept for 12 hours, she laid right there beside me and slept the whole time, not once getting me up to potty. But when she knows I am awake she starts doing her potty dance.
My Rainbow Bridge dog, Shaianne, was crated when I was at work, and for about 6-7 mos I worked 12 hour shift. And she was in there the whole time, when I would clean her crate, I would see she had accidents, but I couldn't blame her, I would have too. I am single and live about 45 min away from my parents, so noone could come and let her out. She did exceptionally well for being in there long. I hated every minute of it, b/c I HATED 12 hour shifts, but I didn't have a choice.
Also, the bigger the dog, the bigger the bladder. So a small dog like a Yorkie or a poodle shouldn't be expected to be crated with no accidents for 8-12 hours either. A bigger dog can hold it longer.
But 12 hours is a very very long time, I would never willingly do it again. Like I said, I hated every minute of it, and have made no bones about it to my boss that I will not work 12 hours again, I will quit first.