There's a John Cage piece called "Organ^2/As Slow As Possible" that is currently being performed on a specially-made pipe organ in Germany. Note changes happen every few years and the piece will finish in 2640.
Also Longplayer[1], played on singing bowls, which will end at midnight on the 31st of December, 2999. It's written to constantly evolve and never repeat. You can stream it!
