The filter cartridges need to be changed according to the need but at least once every season. The running age of the cartridges changes accordingly to the usage of the pool and quantity of dirt in the water. It makes sense to purchase a replacement cartridge at the same time as the actual filter. This way you can change the cartridge when it becomes dirty. Clean the old one and let it dry. This makes the cartridges last a bit longer and you can reuse them a few times.