Some friends of mine set up an open mike with your idea in mind of giving 20 mins to each performer , however their venture now proves to be so successful that some singers book in advance ,if you don't get there within 20 or 30 mins of the doors opening you do not get a spot and many do not get to play. Those that do get 2 songs. Now is that success or have they created a monster and it is out of control. The standard of performers is VERY high. And the room is packed week after week.
So beware!