First..Dances are hard because all the ones people know are overdone a lot. Like to the point were you can't possibly get anyones attention..So I say watch an anime about dancing or that has a dance to it that isn't known too well. It will get the attention of people because they have not seen it yet, or have only a few times ever. Stay clear of Lucky Star dances and the Hare Hare Yukai dance as these are the MAIN two dance things that annoy people more then entertain. As for time limit, we can not tell you that. You have to look it up at your cons webpage or by emailing them. Only they can tell you the max amount of time you can do. I know the con I attened this year so far only allows 1:30 skits so dances are hard to do because anime dances go to an actual song that's about 3min.
Cosplay DA Regular DA
Kumoricon 2014 (so far):
-Sora (Kingdom Hearts 2)
-Alice (Hearts no Kuni no Alice)
-Black&Blue Loli-style Dark Magical Girl O.C.
-Akaito (Vocaloid. Black & Red Ver.)
Dream cosplays for now:
-Tsuruya-san (Haruhi Suzumiya)
-Kagami Hiiragi (Lucky Star)