That depends on multiple factors, but the important thing is that you have patience. The best solar pool heater panels should have the capability of increasing the water temperature by 10-15 degrees. However, it will probably take them several days to achieve that goal. The performance will depend on how hot it is outside. Also, you should always consider the relationship between the heater and the pool size. A big heater can heat a small pool quickly, but a small heating system will have problems with a large pool. You can use a cover for the pool to preserve some of the heat during the night.