I don't know about the longest piece written for organ, but I know what the longest piece to ever be performed on an organ is.
Organ2/ASLAP, by John Cage, is a work for Organ meant to be played As SLow As Possible (ASLAP). It is currently being performed at St Burchardi Church in Halberstadt, Germany, on an organ built for this purpose. The performance began on September 5th, 2001, and will continue until the year 2640. The piece consists of eight pages of music, and each page will take 71 years to perform. One of the pages will be repeated, giving a total running time of 639 years.
Sources: Wikipedia <http://en.wikipedia.org/wiki/As_Slow_As_Possible>, The John Cage Halberstadt Organ Project <http://www.john-cage.halberstadt.de/new/index.php?seite=dasprojekt&l=e>
The longest song ever in the USA was 75 minutes
Eroica, Symphony #3. At the time of its premiere it was the longest symphony ever written.
The longest and only known recorded Pooh is Winnie the Pooh by AA Milne.
Supercalifragalisticexpealidotious
THE LONGEST BOOK EVER WRITTENThe longest book ever written is called "Yongle Dadian". An enclyclopedia with 11,095 volumes and 22,877 chapters.
The longest musical opus ever written is considered to be "Vexations" by Erik Satie. It consists of a short piece of music that is intended to be repeated 840 times, which can take several hours to perform in its entirety.
The creation of sex
"Mahabharata," an ancient Indian epic, is considered to be one of the longest poems ever written. It consists of over 100,000 verses and is a prominent piece of Indian literature.
Ron L hubbards book on sciencetology
The longest song ever in the USA was 75 minutes
The Mahabharata with 74,000 verses and about 1.8 million words. Written by Vyasa.
Eroica, Symphony #3. At the time of its premiere it was the longest symphony ever written.
The longest fingernail ever was...
That would be the vast majority of jazz pieces ever written.
Rapunzel is have the longest someones hair has ever been
The Most Exciting Organ Ever was created on 1965-08-20.