There's a piano piece [1] which is quite short but the original indication by the composer is to repeat it more than a thousand times[2], meaning performances take more than 24 hours when they happen, which is understandably very seldom.

There's also things like John Cage's "As Slow as Possible", which there is a performance of ongoing now. It started in 2001 and you may want to rush to catch the end of it because it finishes in the year 2640.

[1] Maybe by Ravel- my memory and a quick googling is failing me right now

[2] It's some special slightly arbitrary-sounding number like 2347 or something