Finding your flow rate
A flow meter on the return line gives the real number. Without one, a 1 HP single-speed pump on a typical residential system moves 40 to 60 gallons a minute; a 1.5 HP pump 60 to 80. A dirty filter or a long pipe run cuts that a lot, so the actual figure is often the low end.
How long to run it
One turnover a day in spring and fall, two in the hottest weeks or whenever the water is fighting algae. Run during daylight if you have a salt cell or a solar heater, since both only work while the pump is on. With a variable-speed pump, run it at low speed for twice as long: filtration is just as good and electricity use drops by more than half.
When it's not enough
If the water stays cloudy after a full turnover, the problem is chemistry, not circulation. Check free chlorine with the chlorine calculator before running the pump longer.