Your pool water is actually being refreshed every day. Every 24 hours, you will run the pump for eight hours, which pushes the water through the pool filter. If you want to drain the whole pool and refill it, it is best to do so every 5-7 years, in mild weather conditions.
Replacing swimming pool water in hot weather may cause the pool surface to be affected by strong sunlight. However, removing the water entirely after heavy rains or storms may put extra pressure on the pool bottom.