Pretty sure my AC needs a recharge, but I don't really know because I haven't tried using it since last year when I test drove the car before buying. So my AC loss is 0 MPG.
But I presume I would see a significant loss were I to do the crazy thing and refill the AC and actually use it...something I can't fathom doing.
Windows cracked at <40 mph speeds on hot days keeps the interior cool enough, and just the fan on low at higher speeds if it gets hot enough, which it hasn't yet. There's also a slight forced airflow through the vents on my car with the climate control off that keeps the car cool (and warm during the winter) as well. The gray-ish beige interior also seems to stay a lot cooler than the gray cloth in my Civic.
I also like the heat and rarely have a passenger in my car.
There was a MPG loss with the AC running on the Civic I used to have, but I have no idea how much it was due to no actual testing and too much variance and inaccuracy with Torque's fuel economy instrumentation. Plus, I never ran the AC in that car even though it worked fine. I even removed the blower motor for weight reduction...