Stand-up School (Part 2): Practice.
After you have material written, practice will be important. Keeping in mind that most open mics will have a limit of 5 minutes, you will have to time yourself. When you’re timing yourself it is important not to go over your time. A good rule of thumb to have is to make sure you have a solid 4 minutes of material saying it at a normal pace and keep it around 4 minutes, because it’s worse to go over than it is to get off stage a little early. It kind of goes with idea of “leaving them wanting more” or “sparing them from more of your awful comedy.”
Either way it will be beneficial to you. Another reason for the extra minute is for “laugh breaks” which will hopefully go longer than you expect, after you tell a joke or get to a punchline you’re not going to keep going on with your material…which leads me to another common mistake in the performance of a comedian: “stepping on laughs.”