My thoughts, summed up:

For a storyline routine, as long as it needs to be to tell the story in a clear way, get across everything to the audience etc., without having too much unnecessary length which might make it boring

For a non-storyline routine (fan dance, balloon pop, straight strip, etc.) long enough to tease but not long enough that it bores the audience with too much repetition - in my mind this is about four minutes max ...

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