There is no hard set rule when it comes to replacing hair dryers because every device is different. The best way would be to montior your Wet2Dry Baseline. If it changes without any other factors (climate, towel, etc.), then it might be that the hair dryer has lost power. You should also consider replacing your device if there is general wear and tear, rattling noises, exposed wires, or your hair is becoming damaged, frizzy or dull from blow drying.