First, lets review one of our guiding principals. I pulled 6 hours out of the air for a duration, but after a perfectly unscientific but interesting poll on wd3 and some other discussions I have decided to halve that, and set 3 hours for a target. That is not long for a big battle; I am also allowing some slack since 4 hours seems widely acceptable. But it is easier to use up setup, take down and chat time.
The members of a group I had a chance to play Triumph of Will with mentioned that they never got to use some of the interesting features of those rules for rallying troops and bringing them back into action because they only had a pub evening to play the game. I want to avoid that problem.
So, a 12 hour battle in 3 hours of play -- we will have to trip along with an average of 4 times time acceleration. Of course, not every turn will take the same amount of time to resolve, but that average will be have to be kept in focus. Time and motion study time, I guess.