The Great Barrier Reefer by Bongripper is 79 minutes and 23 seconds.

The longest ones I've listened to so far were:
Edge Of Sanity - Crimson (40:00)
Symphony X - The Odyssey (24:09)
Dream Theater - The Count Of Tuscany (19:16)
Shape Of Despair - ...To Live For My Death... (17:19)
Shape Of Despair - Still-Motion (16:29) (this one feels like forever, I love it)

Weakling Dead as Dreams has some long ones.
http://www.metal-archives.com/albums/Weakling/Dead_as_Dreams/13204

Its no too long but its great.. Project Hate- Burn

An excellent 12 minute like song:

Ne Obliviscaris - Tapestry of The Starless Abstract

I like these:

Forests Of Legend - Vektor - 10:16
Dark Nebula - Vektor - 10:28
Accelerating Universe - Vektor - 13:31
Hearts Alive - Mastodon - 13:39
Keeper Of The Seven Keys - Helloween - 13:36
The Day Of Rest - Howl - 10:17
Repangaea - Cephalic Carnage - 12:11
Rime Of The Ancient Mariner - Iron Maiden - 13:36
The Ultra Violence - Death Angel - 10:38
Night On A Bare Mountain - Mekong Delta - 10:24
Suite for Group and Orchestra (Introduction, Preludium, Allegro, Dance, Fugue, Postludium) - Mekong Delta - 21:15
Witheria - Fire And Pain - 10:22

and more I guess ...

You also got a killer song from Dordeduh, song called Jind De Tronuri