It's up to the pool owner. I run mine at night because we are on a "time of use plan" with our utility provider. Electricity is cheaper at night, also I run my aerator every other night or so to cool the water down a few degress.
We run ours during the day to keep the top "layer" of water from getting so hot. Most of the time just removing the cover at night is enough for us to cool the water. We don't live in Arizona though! We have our pump running about 12 hours a day. Whatever works best for you!
I time my pump run cycle to add or subtract heat from the pool. When it is cold, I run more in the heat of the day. When it is hot, I run in the cool of the evening, but for an hour mid day to circulate some chlorine into the spa. I run the waterfall the same way to add or subtract heat and add a mid day 15 min run just to keep some chlorine in it regardless of the weather. I use the Polaris to add circulation in the pool in between, and hour here and there. The Polaris does not help the spa so I had to add that spa time.