You’ll know when this is
You’ll know when this is happening when you become bored. That’s the sign it’s time to stop.
Composition is a different story.
The form of the piece already dictates when you should stop. For example, an ABA form tells you to play the A section once or twice, go to B, back to A then bring the piece to an end. Of course the amount of repetition and contrast is a personal decision but the form establishes both beginning and ending. It’s a nice safe way to say that yes, I have a piece of music here. Now, improvisations can have form as well. The big difference is that you don’t consciously think about putting the music into a predefined shape.
But for some unknown reason, most improvisations do have symmetry – that is they take on a form of their own. I don’t know if this is because of human beings innate rhythm (heartbeat) or what.
Even Zen flute music, which may be the most freely inspired improvisation style around has some structure.
You can hear it in the phrases. Beginnings and endings. Don’t worry too much about them.
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