As a judge, I would rather see an originally-choreographed dance to a song that fits the characters/series source than any of the "known" dance routines. Execution is important, too, so it has to be something everyone in the group can both learn and practice - while in the costumes. My rule of thumb for any kind of performance but especially dance is: if you could do the exact same performance while wearing paper bags instead of costumes, it's not going to get you anywhere. The costumes - and characters - should be part of it.
As far as time limits - you have to stay within the time limit established by the masquerade rules. Don't go over. Don't beg and cry and plead with the director to get 3 minutes if everyone else is allowed 90 seconds. And really, anything over 90 seconds is an eternity to the audience. The judges get the gist within the first 30 seconds, so unless you're planning to surprise and wow them with some twist or really cool acrobatics or something bizarre, they don't need 3 minutes. I don't care how much you like the song or the routine.
Founder and Vice-President, Madison Area Costuming Society, a chapter of the International Costumer's Guild
Strange Land Costuming - www.strangelandcostumes.com