It depends on the vehicle, why it is being left, and how it is prepared before being left.
If it's a vehicle that's in reasonably regular use, then I'd recommend making sure it's used at least once every week or so, and is driven far enough for the engine to get up to full operating temperature. Just starting it and letting it run is better than nothing (e.g. if the car is not road-legal for some reason), but it's much better to actually use it, to get the oils circulating around the gearbox and differentials, and to keep the brakes working correctly.
If the vehicle is properly prepared, it is fine to leave it for much longer (6 months plus) - connect a battery conditioner to keep the battery fine, add fuel stabiliser to the fuel in the tank, and ideally jack it up to avoid flat spots on the tyres. (see other questions on here about preparing a car for long term storage). It'll then need a bit of effort to 'de-winterize' it before use.
Air conditioning needs to be used at least once every couple of months, to prevent seals etc from drying up. The heater shouldn't matter so much though.
In cold weather, it'll take longer for everything to get up to temperature, so you'll need to drive it further.
Bear in mind that if a car is not used much, many parts such as rubber hoses, belts, tyres etc will perish sooner (in terms of mileage) than usual, so will need replacing based on time, not mileage.