I agree with Sebast, of course the answer would be: the more the better. But I think we should try the work out at least one hour. I actually have the same problem, my only time available to practive is in the evening, and some days I m just too tired. One interesting question to ask to the instructors would be, is it the same if we do practice like one hour per day or, for example, do 2 hours every other day? Would that give us the same results?