How often do I need to refill the water tank?

The water deposit capacity of the Optima Steamer DMF is 20 litres. If we consume 4-6 litres of water per car wash we should refill the water tank after every four washes. However the low level sensor in the water tank will go off before using up all 20 litres. When two operators are working with the machine continuously the water tank should be refilled once in an hour. The EST3 has a 40 litres water tank, which allows operating it two times more with one refill.