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It really depends on the car. In a big SUV, it's already burining a lot of gas just to move down the road, so as a percentage of the fuel burned, the A/C isn't adding much. I believe a smaller and more aerodynamic car will suffer more. It also depends on the temperature outside and how cool you want it inside. The A/C compressor usually isn't running 100% of the time, but enough to keep the evaporator cold. The less cold air you need, the less it runs. Although, in some vehicles, the compressor might run 100% of the time.
I drove a Civic for a few years and I remember using the A/C generally dropped my mileage by 5-6 MPG, that was well before I cared about improving my mileage.
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