How long is the world's longest musical piece?

Would you believe 639 years!! The title of John Cage's piece is "As Slow As Possible" which started in 2001 in Halberstadt, Germany. The length of 639 years was decided by subtracting the year 1361 from 2000. In that year the first organ piece was created in the town of Halberstadt. So, the piece should be completed in 2640! Visitors to the church where the organ plays will notice just one note playing as a constant drone. It is rather remarkable if not a bit of a snoozer!

https://universes.art/en/specials/john-cage-organ-project-halberstadt
and https://www.bbc.com/news/world-europe-68209691

This is a recording of the 2006 note change:

https://en.wikipedia.org/wiki/File:Halberstadt_Germany_ASLSP_2006-01-05-17h.ogg

The previous note change had been 184 days before. If you don't want to miss any future note changes, here is the schedule:

https://en.wikipedia.org/wiki/As_Slow_as_Possible#Performance