I find watching the energy consumption for accessories is the best way to make this determination. With summer coming on (with a vengeance) in SoCal, AC on (set to 71°) most of the time, I am seeing about 4-5% of my overall energy consumption going to accessories. So that's maybe a ten mile hit to range. I set the fan to two or three bars. BTW the fan takes some juice too. Try pushing it up with the car stopped, and you'll see your "idle" consumption go from one to two Kw.
The heater is far more energy consumptive, 10-15%, even in our mild winters here.