3-10 gallons per minute. 1.5 - 5 gallons per flush. 20 gallons.
Water is scarce in most of Arizona, and as readers of this 'blog probably understand, water rights out here don't make much sense. "Prior claim to all of the water" is incompatible with both the Nozickian "as much and as good for others" and sustainable use of aquifiers. Lowered water tables destroy streams and change vegetation patterns. If you're still one of those
We're already diverting a major continental river to do absurd things like grow cotton in the desert or wisteria trees and green lawns in Phoenix. (Water is something that everyone needs therefore its price shouldn't reflect scarcity, right?--that's the common logic, and what's good for health care is good for...) Waste water is filtered, centrifuged, and treated with chlorine or ozone to make it suitable to dump on food crops, and water treatment plants have to be upgraded just to meet those standards. Recycling water from tap-to-tap remains a costly far-off possibility.
The craziest waste--aside from the cotton and old men "watering the rocks" in their landscaping--is the use of good, clean drinking water to wash away refuse. Leaving the spigot on while shaving uses 20 gallons of potable water. Showering uses 3 gallons per minute with a low-flow showerhead and ten gallons per minute with the old-fashioned kind. Modern toilets use 1.5 gallons per flush, and older ones use 5. Up to five gallons of drinking water is contaminated with waste and has to be processed every time you flush the toilet.
My recommendations: If possible, install greywater systems, low-flow showerheads, modern toilets, and (in public buildings) flush-free urinals. Turn the tap off while shaving; swirl the razor in a glass. Shower with someone you love or at least someone with whom you're comfortable being nude. And, at least if you're male, for goodness's sake, stop wasting a gallon or more of water every time you have to take a leak. Find a suitable shrub (not a cholla), check for rattlesnakes, and piss outside every once in a while.