Most pool heaters take a day or more to raise the pool's temperature. If they remain turned on at a light level, they are very good at maintaining that temperature. Consider that when you use the pool, it loses temperature, so if you use it regularly, keep the pool heater running.  However, if you have a gas or propane heater that heats pools quickly, you don't need to keep it running. If you do not use the pool for several days, there’s no advantage to keeping it on.