The general time frame for a neutralite refill is about 9 months. The best indicator of when to fill your neutralizer is to shine a flashlight behind the fiberglass tank. This will illuminate the tank and make the neutralite level easy to determine. Your neutralizer is full when it is about 8 inches down from the lip of the tank. When the neutralite is down about 16 inches from the lip of the tank, you will need a neutralite refill.