A/C at least once per week and, like Brett_jjj, I'll use the heater periodically to get the coolant flow through the heater core. You don't have to have the heater on fully hot though, as the heater tap is completely open any time the temp. selector is away from the cold position.
Also, when switching on the A/C, at least initially, I try to do so only at idle speed. That way the rubbing parts, oil seals in particular, aren't immediately dragged up to high speed without a full oil film separating them (that's the theory anyway
).
If you are trying to save fuel (A/C on can be worth 15-20% extra in fuel consumption; you're running the cooling fan(s) as well as the A/C compressor) in most cases you can switch it on only when decelerating (kinetic energy recovery) and still get adequate cooling. I even have a switch on the brake pedal to do this - with some hysteresis via an RC timer, to prevent rapid cycling on a brief brake application. (I would probably use an Arduino were I doing it again and get some more sophisticated switching. Atm, I have to use an over ride/bypass switch for highway use.)
For demisting, if the inside of the glass is misted, using the A/C as well will clear it faster, if it's the outside, it will be better to leave it off. In both cases, using recirculated air will work better than fresh.