In my experience, most open mic nights you will only get to do about 3-4 songs, perhaps more if there are only a few musicians in the wings. If you have 20-25 songs ready you are set to do a full night of playing.
As for being ready, I think you should go out and give it a shot, have a friend video tape it (with decent audio if possible) and then review the tape later for things to improve on.
I started playing at a very young age, so the expectation of musicianship was a lot lower. People expect you to be still learning when that young. Once you are a bit older I think they assume you are accomplished at your instrument.
But since you are playing 3 chord rock, as long as you have fun and play with passion you will be fine, rock is not meant to be perfect, if you play with energy and just keep powering through the audience will enjoy it.
Good Luck!
TAT
