I maybe shouldn’t include the jazz,as there aren’t any “classical” pieces listed. But the line gets blurry. And I’m not ruling out pieces borken up into multiple parts due to the limitations of LPs or CDs.
Free Jazz (1960, Ornette Coleman)
In C (1964, Terry Riley)
I Am Sitting In A Room (1970, Alvin Lucier)
Music In Twelve Parts (1971-1974, Philip Glass)
Metal Machine Music (1975, Lou Reed)
Music For 18 Musicians (1976, Steve Reich)
90 XII C. 9:35-10:52 PM NYC (1984, La Monte Young)
Thursday Afternoon (1985, Brian Eno)
Rothko Chapel (1990, Morton Feldman)
Jesus’s Blood Never Failed Me Yet (1993, Gavin Bryars)
The Sinking Of The Titanic (1994, Gavin Bryars)
Sex (1995, The Necks)
Music For Big Ears (2002, Charlemange Palestine)
There are, of course, many others. These are just what came to mind.