When I was in Italy the AC was on all the time as the temp was about 30C

and the car still returned an average of 45.1 mpg, the best I got was on a run through Austria, mixed use of AC which gave a return of 46.5 mpg.
Acccording to the tech guy from Peugeot, you are getting a slight drag from the compressor any way to stop it knackering up - technical term that

so having it on is not really going to make a great deal of difference.
I did switch it on and off during a trip to see what the difference was, the average fuel only went down by about 0.5 to 0.7 mpg.
Rob.