This Study, courtesy of Ben, suggests that only 3-4 hours per day is necessary for run time. Time of day didn't have much of an effect but a split run time helped some.

I actually did this last summer and averaged about 4 hours per day (2 hour high speed and 2 hours low speed). This turned out to be less than a 1/2 turnover per day and I didn't even notice a difference in water quality. I am not sure that every one can get away with such short run time but it is worth a try.