Perhaps the solution is establishing a standard length of show that is matched to fit the band's degree of success and output. Submitted for the approval of both musicians and ticket buyers everywhere is a set of rules and expectations that ensures neither party has to look at their watch and complain a concert was too short.
20 Minutes or Fewer Maybe it's your first show. Maybe it's your eighth. Regardless, you don't have an album out, though you might have a couple of songs you recorded on your iPhone during practice on your Bandcamp page. Maybe a few dozen of your friends follow your band on Facebook or Twitter or Instagram. You might be playing house parties, or maybe you are the first band on a five-band bill of somewhat notable local acts. The point is, since no one really knows who you are, no one is going to give you the benefit of the doubt. You're only really going to shine for a handful of songs. Focus on the handful of songs you perform best and don't milk it. Leave the audience wanting more, not hating you for playing too long.
Twenty to Thirty Minutes Congratulations! Your band must have just released a buzz-worthy single or EP that has caught the attention of local media, or maybe some bloggers. Sounds like you are going to play all eight tracks of your DIY masterpiece straight through, unless you're going to throw in your unique take on a cool song by one of your influences. Sounds like a great show, but to warn you: If you've heard one Smiths cover, you've heard them all. You might want to wait until your full-length comes out before you take this show on the road.