C30 holding tank should be 17 gallon capacity. Data from my marina uses:Flushes per person/day 5Flush water/flush 1.0 gallonWaste/day/person 0.7 gallonsSo 2 people = 2x5x1.0 + 2x0.7 = 11.4 gallons = 17/11.4 = 1.5 days 4 people = 4x5x1.0 + 4x0.7 = 22.8 gallons = 17/22.8 = .75 daysHope this helps.