And I believe that the only reason it took God a full six days (144 hours) to accomplish his purpose was because he meant it to be a lesson on the Sabbath and making a day of rest and reflection (which lesson is clearly taught in Exodus Chapter 20).
If he had not purposed to use the creation events as a lesson to man, he would have done it instantly (as Augustine believed). I think Augustine may have missed the Sabbatical lesson and that is why he insisted that it did not take 6 days, as this would have implied that God actually NEEDED time to complete his work. He didn't. But he obviously took the time anyway.
Ever notice that the second day is the only day on which God didn't see anything good? So not only did He want to set up the pattern of the Sabbath, but He also wanted us to know that even the Almighty hates Mondays. ;^)