I usually run mine for about 8-10 hrs. per day. You just want to be able to turn over your water at least once per day. I probably don't need that long, but I usually turn in on late morning and off in the evening about an hour or so after I add my bleach. So it runs probably 11:00 to about 7:00 or so.

BTW -- Just an FYI --- evening is the best time to add your bleach to your pool. That way all of it can be used for sanitizing the water and not lost to the sun.