My life partner ran a coffeehouse which had eight acts in an evening. This was not an open mic night, more of a folk-vaudeville show. Every act got 10 minutes, except the featured act, that got 20. She found that some performers would plan ten minutes of songs, forgetting that applause and introductions take time. After someone introduced a short song with a looooonnnnggggggg intro, and everyone else decided to run over time because that act did, she put a dark room timer on a music stand on the stage. She would set the timer on ten minutes. It didn't have a buzzer or anything, but the performer would have an idea of how their act was going.