How do I know when it's time to replace the water filter?

The Filter light on the control panel turns red to indicate it is time to change the water filter cartridge, usually after the refrigerator has dispensed about 300 gallons of water (approximately every 6 months). If water is not dispensing or is dispensing slowly, the water filter may be clogged and needs replacement. Areas with high lime content in the water may cause the filter to clog more quickly. (Page 21)