It took two weeks (almost exactly) to go through my first tank of gas in my new Prius.
That's not the real story, however. What was impressive is I went 465 miles and the fact I only had to put 9.5 gallons of gas into the car. That's a lot of miles per gallon! Yea!
This tank included back and forth to work, a trip over Highway 17 to Santa Cruz for a consulting job and basic normal driving around the area. This was all on the normal hybrid mode (although I have used the EV or stealth mode from time to time just because it's cool).
For the next tank of gas, I'm trying the Prius' Eco mode. Eco mode is supposed to maximizes the fuel savings across all driving conditions. It changes the throttle on the car, reducing it's response time, as well as some other fuel efficiency changes. I've read reports the Eco mode improves efficiency by 10 to 20 percent. We will see if that's the case with my driving.
But, in any case, if I continue to get this kind of milage, I'll be happy!