That depends. Somewhere between 12 and ∞ mpg. ;-)
~20 mpg when running in charge mode and >300 mpg (the on-board computer maxes out at 300) for the first 15-25 miles in EV mode (starting with a full battery). Roughly 70-80 mpg for the first 75-150 miles in hybrid mode (again with a full battery). Over longer distances, hybrid mode yields 55-65 mpg. The lowest instantaneous mileage I've seen (flooring it with no battery) is 12 mpg.
Over my first 6K miles, my average is around 60 mpg but my day to day driving has been 100% battery since winter ended so that helps.
Those numbers are carrying only the driver for short trips and a driver, passenger, and luggage over longer distances. All in auto mode for the handling and performance setting.
In general, you should expect better mileage in warmer weather (over 45ºF/7ºC). 65-75ºF (18-24ºC) seems to be a sweet spot. Haven't isolated whether that is due to the operating temperature or it is a result of not using seat heaters or AC.