How Many Times Do You Have To Listen To Understand A Piece Of Music?

I don't think you have to objectively "understand" a piece of music to absolutely enjoy it right away, from complex jazz or classical to a simple tune. If it feels good, it's good. I was a Beatles and Dylan fan in the 60s when it was new and would sit stunned when first hearing some new thing...saw Monk at 13 when he opened for Peter Paul and Mary at the Hollywood Bowl...didn't know any jazz, no clue who he was, got my mind blown. All that resulted in a decades long career playing music for a living.

Rap or hip-hop: about 15 seconds.  Most contemporary pop/rock: first listen.  Selected pop artists (mostly indie, e.g. Kate Bush): a couple of listens.  Springsteen: ain't gonna happen in this lifetime.

If I’m critically listening, not hearing at a holiday party at someone house, once.  Some music grows on me like an earworm.  My kids music sometimes.  I’ve heard it so many times I know every word by heart but I wouldn’t put it on my playlist.  

There are many composers that people I respect tell me that I ‘have” to get.  Schoenberg and his school are the most frequently cited.  So after years of trying I finally understand the music, but I still don’t much care for it

About 16 bars is usually enough for me to know whether I want to listen further.