Every two days
We have no gauges on our P42 but know our capacities. Our general rule of thumb is with 4 people on board, if they all took showers and we ate on board and had dishes to wash, then we need to take on water and pump out the holding tank every 2 days. It works.We have never run out of water or overflowed the holding tank [35 gal]. We have run a water tank down to nothing but have always had enough left in the other to get us through. [I try to balance our water use. If both tanks are full, I start on the port tank to eliminate the lean to port from the weight. After the first round of showers, I switch to the forward tank. I stay on forward until I think it is about empty, then switch back to port tank. Sometimes forward runs out before I switch back to port.] I think gauges would only tell me what I already know: its time to take on water or its time for a pump out.