There's only two shows whos intros I actually like and those are from "Sanity not Included" and "Game Theory." Mostly because they're very well edited and they have their own original tunes. And the Sanity one is ~28 seconds long. I like intros that are more "show-based" not channel-based. In other words, a specific intro that only plays on that particular series, not every video on their channel.
But, I'd say 10 seconds or less. I honestly prefer around 5 seconds if you must have one.