theDarkSite wrote:That means that you can try even that your AMV is over 5 minutes

Actually, YES.
You can always TRY anything.
Whether it gets PICKED is up to the judges. If a story or theme, or the lyrics of the song make you feel you MUST go long, them make your best judgement. Holding the audience on the edge of their seat for 5min 40sec works. Just imagine them exhaling after it finishes, and saying a collective 'Whoa, that was INCREDIBLE' and no one was looking at their watch.
If on the other hand, two minutes into the AMV, people are whipping to writstwatches and texting, and thinking 'Oh, no, it's not even HALF-OVER yet...' well that's 'Duration not so much, pleeeez,' and the judges may not select that entry.