If singing a long ballad is an academic exercise to you, it's going to sound that way to an audience and you're going to lose them. If it's something about which you're genuinely passionate, that passion might translate into audience interest. Or it might not. Audiences is funny creatures.

One thing for sure though; unless you're a household name or are on a first name basis with every member of your audience, don't start out with a long ballad. Toss a few easily digested songs out to warm the audience up first. Then, if you haven't yet been booed off stage, try the ballad. If you try a long ballad early in a set and it doesn't go over well, you'll never get the audience back.