Like a Wall in Which an Insect Lives and Gnaws by Bull of Heaven, it's 5 years, 8 months, 13 days and 20 hours long, but since I haven't listened to it, it doesn't really count.
Second place would be Jethro Tull's Thick as a brick, which is around 44 minutes long.
Third place is Cassandra Gemini by The Mars Volta, it's around 33 minutes long.
Putting back fun in funeral.