I get around to lots of open mic nights. Just last night I went to one at The Dancing Goat in the town of South Orange. I had never been there before. I was just sitting at my computer last night, and somehow ended up on njopenmic.com, where it was listed, and then just got up and went. It's a nice place, and it full of lots of young people (not quite urban, yet not quite suburban -- if you care). Full list of acts, runs from 8-11 went by fast. When I got there it was standing room only and the sign up was almost full, but I got a slot toward the end. I got on stage at around 10:20. By this time, the crowd had thinned, and busboys were starting to move into full force clearing tables. So after I played I felt rushed to pack up and get out. You see, at the Sanctuary, my usual Tuesday haunt I usually prefer this time slot, anywhere between 10-11, because that's usually when the biggest crowd is there. So you see, when I played last night at 10:20ish to a mellowing-out crowd, it would be the same time, a day prior, when I would be playing to a vibrant crowd at the Sanctuary. And then get sit and hang out and chill for at least another hour, because it continues until at least midnight or 1am.
But every place is different.