We have 2 30 gallon grey tanks and a 30 gallon black, the black tank will usually last the 2 of us 5-6 days. The Galley tank will usually last at least that long as we eat on paper plates and only wash the dishes from food prep. The shower grey tank will last us 6 Navy showers, which might be 1 day if the DW is running around in the summer and "glowing" (sweating to most of us)... :D
Don't really pay any attention to the gages anymore, they are notoriously inaccurate and we've learned how to tell when the tanks a getting full by the sound
