It depends on where you are relative to the reservoir. How long it would take for the elevated water tanks to run dry will, of course, vary from one place to another.
Even if the power is out chances are excellent that the water will continue to flow no worries.
The water will continue to flow, but the longer the power is out, the more likely some sort of backflow compromises the system. We've had a couple of "boil water" advisories around here after power outages. Of course, that's a CYA reaction to marginally less-safe water, not an indicator of a serious threat, but it does let greeblies (a new word to me, but I really like it) into the pressurized, post-treatment part of the system where they're not supposed to be.
I didn't have any water problems when the power went out, but some parts of the county had extremely low pressure. A lot of the people I talked to had no idea that anything was wrong. In a crisis, with panic adding increased use to reduced capacity, there's a potential for delivery problems.
Sewerage, too: toilets are one piece of household technology that are unlikely to fail in an emergency.
Absolutely. That's why one of my priorities in emergency water is having enough on hand to refill the toilet tank.
A very interesting exchange between you two about potable water. Thanks for the tips, especially about the hot water heater. I never thought of that one.