The time it takes to heat a pool with a heat pump pool heater depends on several factors, including the size of the pool, the initial water temperature, the desired temperature, and the ambient air temperature. Generally, it can take anywhere from 24 to 72 hours to raise the temperature of a pool by 10 to 20 degrees Fahrenheit. Keep in mind that heat pumps are designed to maintain a set temperature rather than rapidly heat the water, so for the best results, it’s advisable to keep the heater running consistently during the swimming season.