I think a lot of people are missing the point of this, so let me re-iterate in more clear terms.
For those of you who ask me how long your track can be, etc... it needs to be as long as you think it should be, or could be.
This project is for those moments when you hear song and think to yourself "Wow, this one part of the song goes well with such and such anime! I wish the whole song worked, though."
The point is to make an AMV to that part of the song that struck you.
I don't know how else to put it.
If it's just one line of a song, and lasts 10 seconds, that is fine.
If it's an entire verse and is a minute long, that is fine.
This is so very very very open ended, so just edit whatever suits your fancy!
Not sure how else I could put it to make it simpler.