Great idea, Jeff, and sweet playlist.
I plan on making at least a few of these, as it's rather fun. They're going to be broken down by theme
Timmy's Playlist of Really Long, Somewhat Pretentious but nonetheless Epic and Awesome Progressive Rock Songs- Cassandra Gemini by The Mars Volta
This sprawling 30 minute epic is simply face-melting. You want jazz flutes? You got it. Trippy noises? Check. Pretentious weird prog noise? Of course. Saxophone? Check. Mile Davis-esque editing and assembling? You bet. One of my absolute all-time favorites.
- Atom Heart Mother by Pink Floyd
Ok, they didn't get to finish it, and if you notice towards the end it repeats itself from the beginning, but this 24 minute song is all kinds of awesome. RUMMAH TIKKAH, TOO DOO DAHHHHHH! The rest of the album is kinda weak though, so the song being 24 minutes is a very good thing.
- 2112 by Rush
The quintessential long prog song. Neil Peart + drums = eargasm.
- Larks' Tongue in Aspic Parts 1 and 2
Part 1 comes in at around 13 and 1/2 minutes, and part 2 comes in around 7. Combined, they make one of the most innovative rock songs ever created.
- Tarkus by Emerson, Lake and Palmer
Who doesn't love a moog modulator synthesizer?
- Echoes by Pink Floyd
The version of this song from "Live at Pompeii" is an awe-inspiring piece of artwork. The one from the original album (Meddle) is awesome too.
- Tetragrammaton by the Mars Volta
Clocking in at 16 minutes, this is my all time favorite song. It's the kind of song you don't fall in love with at first, but each time you listen to it, it becomes better and better, which is a pretty typical phenomenon for prog music.
- Achilles Last Stand by Led Zeppelin
Honestly, not one of Zeppelin's strongest songs, but it's still great, and at 10:28 min. it's long enough to qualify the list, and I wanted these guys to make the cut

- Bitches Brew by Miles Davis
Ok, I realize this is technically categorized as jazz, but it's amongst the most progressive jazz you'll ever hear. It's full of weird awesome stuff too. Clocks in at 27 minutes.
- Just kidding, there is no more. This playlist is too long as it is. And you thought I was going to put a Dream Theater song here, HA!