Intros for us were hard because we wanted something catchy and fun and still explain what we do. We tried 10 secs, 20 secs, and we just decided to put something non intrusive and something that just gets out of the way fast, but is still uniquely us. So we settled on 4 secs of bad singing and our logo.
However, I have seen some intros that were like 15 secs long and I've replayed them because they had a rockin' soundtrack and cool graphics! But I guess it all depends on what you think your audience can handle!
One more thing, yes we had intro fatigue! We always felt there is one more thing we want to change, but in the long run it's better to leave them wanting more!