The barn I'm at (and about to take over) has posted hours of 7AM-9PM. None of the boarders have had a problem, except one that came up late to take her horses without paying board. She had other issues though, obviously, haha. The BO isn't strict with the rules though and has had people over late or even overnight.
I've been combing through the rules to update everything before I take over, and I went ahead and added that if anyone wants to come outside of posted hours to arrange it with me and my husband first, as we will live on site. As a boarder, I'd refuse to stay at a place that restricted me from seeing my horse, but as a BO, I'd worry if someone started messing with horses and property at odd hours without my knowing why.