If it's too long to be accurately heard and remembered when you shout it into some girl's ear on a crowded dancefloor, it's probably too long to be a branded domain name.
If you're talking about EMD stuff, you've got a lot more latitude, although I have an unconfirmed suspicion that as you start stretching the EMD title length, you get closer to what G considers "low quality" site territory.
"How Long?" is kind of an eternal question.
People have been wondering this since at least 1974, as evidenced by this video:
[ame]http://www.youtube.com/watch?v=qGHiGLqhFfY=embed[/ame]