Easily one of the greatest achievements in musical history was when shouty adolescent punk heroes NOFX decided they had "done enough short songs, time for a long one" and got together to write an 18+-minute epic, thereby making their entire genre look inferior and childlike by comparison, while still managing to sound like Blink 182 and Green Day the whole way through. Listen and prepare to be amazed:
No love for "American Pie"? Actually, I get it. I love Don McLean, but it's not one of his top songs, and not actually my favorite long song. I'll also give an honorable mention to Arlo Guthrie's "Alice's Restaurant" and Harry Chapin's Sniper, which ranks just ahead of and behind "American Pie" respectively on my list of favorite long songs.
Off the top of my head, my favorite songs longer than 6 1/2 minutes (rounds to seven, excludes the dozens of five-six minute songs I like) are:
Moody Blues — "I Know You're Out There Somewhere"
Paul Kelly — "From Little Things, Big Things Grow."
The Pogues — "And the Band Played 'Waltzing Matilda'"
Bob Dylan — "Rosemary, Lily and the Jack of Hearts."
And I need to give a special mention to Jethro Tull's "Thick as a Brick," which was written as one album-length song, though due to the constraints of technology at the time, is almost always presented as two tracks, as below. It is an my favorite album of theirs.