The object is to turn over the water at least once a day. That means running 10,000 gallons through your filter. It all depends on how many gallons a minute are being pumped through. But usually 8 hours / day seems to be enough.
Lots of people have different opinions on day vs night. Day, when people are using it, keeps it cleaner when they are using it. Running it at night distributes the chlorine better without UV breaking it down.