... about 5 gallons in my case. I can do it in 4, but give me 5 and I'm happy.
This weekend, I managed to wreck my tankless hot water heater. I flushed it with vinegar as per the manufacturer's directions and managed to cause a leak. San Diego water is more like chunks of limestone than an actual fluid. It causes all kinds of corrosion and calcium buildups.
As far as I can tell, the pipes in the heat exchanger had corroded and sprung leaks. Water didn't leave because calcium deposits had plugged the holes. When I flushed the system with vinegar, I managed to unplug the holes. Turning the water back on led to drip-drip-drip-drip-drip. Since I did this exercise on the Saturday when plumbers were not available, we had nearly two full days without hot water.
I've now taken two showers by heating a large pot on the stove and using it to heat up a 5-gallon bucket of water. I then stand outside with my bathing suit on or stand in the shower and dump bowls of water over my head to shower.
It's like going back to the 1800s. We're very retro here in the Catican Compound.
In any case, the 5 gallon bucket was plenty of water for me. Now I know.