I can chime in here with some experiential insight.
For the last few years, I have been the regular host of the longest running Open Stage in NJ. We have a regular Friday night acoustic concert series (The Minstrel) in Morristown, NJ and the second Friday of most months is our Open Stage.
We start at 8:00pm and go until 11:30 with a short break in the middle. There are a total of thirteen 15 minute performance slots. Six of those are available by advance registration, one goes to a volunteer and the remaining slots are distributed via lottery 15 minutes before show time. I tell newcomers about the general house etiquette before we start. Be in the audience for the other performers, be ready, be in tune, be quiet and respectful, etc...
We are an extremely enthusiastic, nurturing group of folkies and our Open Stage is a fantastic place for anyone's first performance. You can be nervous, fumble, make mistakes... and we will applaud like you were Eric Clapton. We tend to attract a fairly broad range of performance levels, from beginners to professionals. It is fairly well known that we sometimes book opening acts for our regular concerts from the Open Stage.
Over the years, the regulars have gotten into the habit of giving up songs to anyone that doesn't get a slot in the lottery. I also keep things moving quickly enough so that I can sneak one or two people in for one quick song. We actually managed to get 18 different performers on stage one night. (Another night, it just happened that everyone that signed up had really nice guitars, so as I introduced each performer, I also mentioned the instrument they were playing.)
If any of you live within driving distance of Morristown, NJ I would love to see you at our venue! Ukuleles are always welcome (I actually threw a Uke festival there last year and have started booking the sequel for this year).
The Folk Project' main page can be found here:
http://www.folkproject.org/index.shtml
The Open Stage instructions are here:
http://www.folkproject.org/minstrel/openstage.shtml
Scooter