This is something I wondered about as well. Having had a pool for 2 summers now I have learned a few things. This summer I have been able to run my pump 4-5 hours during the hottest days (110-115 deg) of the year and water temps in low to mid 90s.
Last summer I ran the pump 8-9 hours because I didn't know any better. With the excellent help of mas985 I realized I was getting a full turnover in about 2 hours and really was running up my electricity bill for no reason.
However, to my mind that is a long time to leave hot stagnant water. So currently I run 1 hour at 9 AM, from 1-3 PM, and 6-8 PM. This gives me peace of mind. I don't run at night as it is my understanding algae requires some sunlight to grow quickly, with a cell doubling time of 3-8 hours. Thus with about 4 hours between stirs I feel comfortable I am keeping growth in check.
When the heat dies down next month I will likely go back to 4 hrs per day, dropping one hour at the end of the day. Last winter I ran it 2 hours per day and it was perfectly clear. I could probably get away with one hour with water temps around 50 deg.
Save some major $$ - cut way down on the run hours.
Eventually when my pump dies I will be switching to an Intelliflo VS. However it's tough to justify outly with my electric rates around 13 cents kw/h.