For me, the primary criterion when it comes to whether something is an epic or not is the arrangement, not the length. Length doesn't really cut it. Say you have two songs, both with two 16-bar verses, three 16-bar choruses, a 16-bar intro, 16-bar solo, and 8-bar outro. That's 120 measures. But say one song is at 6/8 time at 150 BPM and the other in 4/4 at 70. The one would be just 2:24 and the other would be 6:51, but there's no difference in the actual scope of the song, the same way that if ACOS was written at a 25% faster BPM, it would be just as epic (though likely less epic in the superlative sense of the term).
So, for me, what defines whether a song is an epic or not is whether it adds to the typical arrangement of a song. A typical song has 2-3 verses and 2-4 choruses, along with an optional bridge, intro, outro, small instrumental break (typically before verse 2) and larger instrumental break (often consisting of or including a guitar or keyboard solo, sometimes even both). A song that has all of these--3 verses, 4 choruses, a bridge, an intro of some substance, an outro, a solo, a riff break after the solo, and a verse break, would be about the longest non-epic song you could get (especially if at a slow BPM, at which point you might be pushing ten minutes). Once a song starts tacking on other things--the extra-long instrumental break, two or three solo breaks, a full-on overture up front, multiple styles of verses and choruses, bridges that morph into entire movements, etc. etc. it's an epic. How epic it is varies--ACOS is obviously further along this continuum than Endless Sacrifice is. Note also that The Killing Hand, with its eight verses and umpteen instrumental breaks, is decidedly more epic than, say, Outcry, whose single stretch of the traditional song format is the extended instrumental section. A song like, say, TROAE is basically the dividing line for me. Most of the song is obviously very standard structurally, but there's the extended bridge with the tempo change, a unison break that leads to a tradeoff solo, a fairly long intro, an outro that introduces a new theme, and even the chorus has an interesting arrangement, kind of split into two halves. Could go either way on it. But anything over the TROAE line is "an epic" for me, though how easily each song earns that distinction obviously varies. Clearly the 20+ minute songs tend to be an extra order of magnitude above the rest, but it's equally erroneous to put ANTR into the same "just a song" category as Burning My Soul as it is to put Take The Time into the same "epic" category as ACOS.