The Reservoir Holds 3.5 gallons / 13.2 litres of solution.
The frequency of fill rate and nutrient replacement will depend on multiple factors such as plant size and quantity, ambient temperature and watering frequency.
As a standard - watering 1.5 minutes per hour with plants at a harvestable stage will require a top up every 4 to 6 days. Replacing the full reservoir with fresh nutrient every 2 weeks is ideal for optimum yields.
If you are leaving on vacation your can reduce watering frequency to as little as 1 minute per 4 hours and maintain a functional water level for up to 14 days depending on the size of your plants.