The longest recorded piece of music used to be by a noise band called Bull of Heaven, "The Chosen Priest and Apostle of Infinite Space." It's more than 2 months long. They also have other works that are multiple weeks long. (see Link) They have however bested this with a song that is 50 000 hours in length, that's more than 6 years of continuos song! One must note, however, that Bull of Heaven does not write actual music but rather repeats sound effects at very slow speeds for days on end.
The progressive rock band Green Carnation made a 1:00:07 song called Light of Day, Day of Darkness, the album had the same name. This song was the only song on their album.
The progressive rock band The Flower Kings's song Garden of Dreams is nearly an hour long, at 59:24. They also have recorded The Truth Will set you Free, which clocks in at 31:03. On the same album, Devil's Playground is 24:45. The Flower Kings is well known for very long studio tracks.
Spock's Beard has a few long songs, such as The Great Nothing (27:02), and The Water (23:10). The ex-lead singer Neal Morse's solo project also has some long songs, such as The Door (29:13), The Conflict (25:00) and The Conclusion (16:24).
Rock band The Dragon Cult are known for their extended studio recordings, including Black Ghost Train (38:05) on their debut album, and for their marathon trio improvisation jam sessions at live performances.
Dream Theater also has a song that is 42:04 called Six Degrees of Inner Turbulence. They are also known for long studio recordings.
The prog rock supergroup Transatlantic also has quite a few long tracks, such as Duel with the Devil (26:43), Stranger in Your Soul (30:00), All of the Above (30:59), and The Whirlwind (77:54 *This track is broken into 12 parts on the studio album*)
The progressive rock band Magellan has a track "The Great Goodnight," which clocks in at 34:45.
Genesis' track Supper's Ready is 22:52.
Emerson Lake and Palmer's track Tarkus runs for 20:40.
Mike Oldfield, known for long tracks, wrote Tubular Bells, which is nearly 50 minutes long (48:57), but was broken into 2 parts on the LP. Hergest Ridge is 40:14 seconds, and was also broken into 2 parts over both sides of the LP. Ommadawn is 36:41 (same deal). Incantations is 72:44 (broken over two vinyl discs). His song Amarok is also quite long at 60:04.
Keith Jarret has very long songs, like the Vienna Concert, which can be 45 minutes long.
The band Green Carnation have an album called "Light of Day, Day of Darkness" that consist of just one song and it clocks at 60:06 (1H and 06sec).
Manowar's "Achilles, Agony and Ecstasy in Eight Parts" is 28:38
The longest solid Dream Theater song is Octavarium (23:59). The DT song "In the Presence of Enemies" is over 26 minutes long, but is broken into 2 parts on the studio album. A Change of Seasons is also quite long, at 23:06.
The progressive trash death metal band Meshuggah has the song I (21:00) and Catch-33 (47:09). Although Catch-33 is broken into 13 parts, they ARE part of 1 complete track.
Some Pink Floyd Songs are long :
Atome Heart Mother Suite (23:35)
Porcupine Tree - From The Sky Moves Sideways, the title song is split into two parts that add up to over 35 minutes. Additionally, their album Voyage 34 is composed of 4 phases over 10 minutes long each.
But there is another new band (progressive blues rock shredding) currently known as S&S Co. they have a song with no official name yet that is 39 minutes and 24 seconds. "One of those few bands that makes it up there with the classics such as Hendrix, Zeppelin, Beatles, Rolling Stones, Pink Floyd, and all": Rolling Stone Magazine. Not too many people know about this so that's why I'm sharing this with you.
Also the song created by the band "SLEEP" made a song called DOPESMOKER which is 60:03.
Iron Butterfly recorded In-A-Gadda-Da-Vida at 17:05
Dream Theater's Metropolis Pt. 2: Scenes From a Memory is about 74:36
Deadmau5: 4x4=12 (continuous mix) 69:54
Dream Theater - Six Degrees of Inner Turbulence (42:00)
Dream Theater - Octavarium (24:00)
Dream Theater - A Change of Seasons (23:06)
Mountain Jam by The Allman Brothers (22:00-45:00)
Mirror of Souls by Theocracy on their album Mirror of Souls is 22:26.
Yoo Doo Right by the krautrock band, Can, was originally six hours long. It was cut down to twenty minutes when it was released on the album Monster Movie though.