Mostly a matter of preference. Some places have peak hours where the rate is more expensive, others don't. If you have solar panels, you want to be circulating water during peak sun hours for maximum heating benefit. If you have solar panels and your pool is too hot, you can run the water through the panels at night to cool the water down. As long as you run your pump long enough to turn the water over at least once per day, you will be fine. Some people do 24/7 which is really unnecessary and wastes electricity. Some people do a couple of different sessions per day -- each about 4 hours. Some turn it on when they get up in the morning and off when they go to bed. I do 12-8 to maximize heat gain from my solar panels. There is no one right answer.