The length of your set(s) is dependent on:
Whether your performance requires a certain length (i.e. theatre, comedy, performing the entire album beginning to end for an album release)
Whether it’s a seated or a standing show
Whether there’s an intermission
Whether there’s a cut-off time that show has to end
Whether there is another act sharing the bill
Who your audience is (are they children who don’t have much of an attention span?)
Work with the Host to discuss a plan - they will know their audience best and what works for set length in their space. A general rule of thumb is between 30-45 minutes, and saving something for the big finale if there’s an encore. But you also want to prepare for anything in the moment, and use your judgement during the set - for instance, does the audience seem antsy on your 7th song but you still have 4 more on your set list? If that’s the case, maybe cut off a couple of songs and finish on a high note.
If you are opening for another act, you may wish to keep your set on the shorter side. Discuss with the other performer how long their set is and adjust accordingly.
If you’re the only act of the night, you have the option of doing two shorter sets with an intermission. Ask the Host what their preference is.
If there is more than one performer, having an intermission is a great way for the audience to take a break, use the washroom, fill up their drink and chat with fellow guests. In the introduction to the show, the Host is encouraged to let the audience know about the intermission and so they can just focus on the performance when it’s happening.
The more performances you do the more you will get used to reading the room and fine tuning (pun intended!) your set length for each show.