It depends on how much your local weather, route and driving habits keep the AC compressor and ICE running.
I live in Phoenix where it gets pretty hot in the summer. Over the last couple of months, I was getting about 42-44 mpg on my daily work commute, which is mostly city driving. This past two weeks, the drive time temperature has been low enough that I have been driving with the AC off and windows down, and I have gotten 54-57 mpg driving the same route.
I have been working a lot of overtime, and the traffic is lighter after rush hour, which may account for part of the higher mileage. But I was getting 52-54 last spring.
For highway driving, the AC seems to make very little difference; probably because the ICE is running most of the time.
I have a 2002 classic Prius. This may not hold true for 2004 and newer models.