Does length of workout matter?

depends on what your goals are and how much time you have.

but here's a great article that explains fat loss http://www.t-nation.com/free_online_article/sports_body_training_performance/the_hierarchy_of_fat_loss

for fat loss, i dont think it matters if you break your workouts up into 2 or 3 because the benefits are cumulative so it's all going to add up to an hour for the day.