How fast do they cycle the water? Say if i uuse a 5gal container. Will it run out before I have time to wash? And is there a huge issue if it runs dry? Like if the water runs out before I can refill the holding container?
Its very relative to the particular system you build or source. Using the pump supplied with the Helton kits for example, my wife and I can each shower twice on 5 gallons of water. These are not 20 minute soak forever showers, rather get wet, wash off the dust, soap up and a good rinse. Now, using a higher volume pump, you get a more "comfortable" shower per say, but you can burn there a lot more water. There is no right or wrong answer, you just need to decide your needs and go from there. Its very common for people to buy a comercially built heat exchanger and build their own water pumping system from there, using something like a SureFlo pump and a shower head from the RV store or Home Depot, etc. Each has its own benefits. The Decker kit for example comes with a much higher flow pump than the Helton kits, if your often camping near endless water supplies, there is no reason not to use the higher flow setup, just do your best to minimize the wasted water by turning off the pump. In our case, we leave the pump on and simply shove the shower head into the Scepter can, thus just re passing the water through the same closed loop, minimal wasted water.
Regarding water sources, I would urge that any using an open water source such as a river, lake or stream to be very environmentally conscious. Soap/shampoo selection, waste water run-off and parking your vehicle so close to water source, all need to be done with forethought. Due to these reasons, my wife and pretty much just use our shower out of a Scepter can. If we run low on water, take a short trip with the Scepter can to the water source and fill it up, leaving the shower a safe distance from a otherwise pristine water source.