The thing is, though, it all goes back to following cues. My now ex-best friend (ex for many reasons, not just this) simply could not do it. She had hard and fast "rules" that would end up causing more problems than they solved. Case in point, relevant to this particular one:
A big group of us went to La Nouba for my birthday a few years ago. One girl in the group had some connections and arranged a VIP backstage tour and seating up by the lighting booth. It was a fabulous evening. Afterwards, we were standing around in the parking lot talking.
Now, my ex-best friend only knew me, my dad, and one other person in the group. Everyone else was old theater friends of mine and my dad's, whom we hadn't seen in a while (including the girl who made the arrangements).
At one point, one of the theater guys said something about needing to head home because he had work the next day. And then he kept right on chatting for another 20-30 minutes. That's just how it's done in that particular group. We all keep mentioning that we should probably go home, but nobody really wants to, and it's not a serious comment until someone actually starts heading for their car.
Well, my ex-best friend got furious with me. She started yelling at me, in front of the entire group, about how the guy had said he needed to leave and I kept him talking, and how dare I be so rude! Needless to say, it was a bit shocking to everyone, and really killed the mood.
So yeah, it's great to have rules like don't stay too late. But if you're in a big group of people who ALWAYS stay late, don't be that person trying to rush everyone along because in your mind, it's too late. Not saying you would do that, but just something for all of us to keep in mind.