It all depends on who the act is that's playing and the day they're playing.
I've seen it twice, once a couple weeks back when Survivor played, and yesterday with The Beach Boys.
With Survivor, there was not much of a line. They filled up most of the concert hall, but it was a pretty relaxing, fun show (although I only knew one Survivor song - Eye of the Tiger

) with a moderate crowd.
But, a TOTALLY different story with the Beach Boys.
I'm not exaggerating when I say this: people started lining up at 2pm for the 5:45 show...a LOT of people. By the time the show started, the line was...get this...back out to the AFRICAN OUTPOST. Literally. It was CRAZY.
I had no idea the Beach Boys were still this popular. The crowd level was unlike anything I've ever seen at EPCOT. But think about it: it was a Sunday, so tons of locals (like me) were there to take advantage of F&WF; it was about THE most PERFECT day, weather-wise, you can possibly ask for; it's F&WF, so it's going to be crowded to begin with; and the Beach Boys were playing.
For much of the BB show, you could hardly move through the AMerican pavilion. I couldn't believe it. So just consider who the act is that's playing and plan accordingly.