A hook can be any lyrical (or musical) element that sticks in the audiences head. That being said it's usually on the short side because the longer it is the harder it is to memorize in full...and you see where I'm going...it completely negates the idea of a hook in the first place.
If you can get you hook to be long, and still so musical to the point where the audience can remember it (or perhaps cannot get it out of there head), then more power to you. It's quite a bit harder to make a long hook, but it can be done.
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