Ten to fifteen minutes a side should be expected. The Group goes first, then the Initiative, then the Group finishes. In finishing, the Group should only address things that came up in the Group's opening or in the Initiative's arguments and should not introduce new matters. If they introduce new matters you may wish to consider allowing the Initiative 3-4 minutes for a rebuttal or excluding that argument. Everyone will then write an arbitration award.
Nothing is an award just as much as something is.
Each side has the burden of proof, of making it more likely than not, for the evidence relating to the things they want. The initial practice run should take three hours or less to organize and practice. You are also expected to practice the parts of an arbitration and pay attention to what is going on. You are not graded on your public speaking skills and not expected to bring anything new or creative. Call the one of the local Better Business Bureaus.