In my opinion, if you're going to have a long brickfilm (and I'm talking like 20-40mins), it all starts to rely on the plot. Animation, indeed, will only keep the viewer interested for so long. But if you have a really interesting and compelling plot, then I believe the brickfilm will keep the viewer interested for however long it is.
Of course, your average youtube viewer, or your average movie watcher for that matter, may just not be interested in watching a 30min LEGO FILM. I think the community would take it as something great, if you really did make something that good, and you may win over a few "average" viewers who are not experienced in Lego films as well. But I still think the mass of them would get bored with it, no matter how interesting the plot was. Now of course if your film is somewhat interesting visually, and I don't mean perfect animation necessarily, but rather a fancy set or fantastic special effects, then this might suck the viewer in and keep them interested in addition to the plot.
So all in all, I do not think there is such a thing as a brickfilm that's too long, I think there is such a thing as a pointless, visually uninteresting, brickfilm that could be too long for its own good. So if you're going to make a long brickfilm, I suggest you get together a decently interesting concept/plot, and I suggest you think of ways you can make it visually interesting as well...that is, if you want to expand far beyond the reach of the brickfilming community.
Into the darkness once more. I walk into the unknown. To a far better life than I've ever known.