Once bands (DT and others) go past the 20 minute mark, they have a hard time keeping the song together. For example, I consider the first couple of parts of ACOS some of the best stuff they've ever written, but the rest of the song is so disorganized that it takes the solid closer to make me finally remember why I almost love the song. 6 Degrees is decent in and of itself, but it's not so much a single track as it is 8 separate tracks thrown together with a broad musical and lyrical theme tying them together. Octavarium is probably the one true barn burner that I can honestly say I enjoy from start to finish, but even that could have used a bit of paring down, most notably in the too-long Floydian opening.
With that being said, DT usually excels at the 12-15 minute mark, with true works of beauty such as Home, Trial of Tears, in the Name of God and Learning to Live. Sometime they do well beyond it, with Count of Tuscany being a prime example, but for the most part, I'd like to see them scale back on their epics for a couple of albums and offer more shorter, but still quality songs.