You’ve parked your car, and are sitting inside waiting. Maybe you’re early for work and don’t want to head into the office just yet. Maybe you’re waiting for your spouse to get done in the convenience store. You don’t want to waste gas, so you turn off the engine, but it’s warm, so you leave the blower running. You know this pulls current from the battery, but just how long can you leave the AC on while your engine isn’t running?
Actually, this is a trick question. You cannot run the AC without the engine running. The air conditioner compressor and clutch are operated by the serpentine belt that runs your other accessories (your alternator, for instance). If the engine isn’t running, then the belt isn’t turning. That means your air conditioner’s not actually on. You cannot use the air conditioner if the engine is not running. It’s as simple as that.
What you’re really doing is running the blower motor. As long as there’s a charge in your battery and you’ve got the key set to the on position (some cars will let you run the blower in the ACC position, too), the blower will continue to operate. However, it’s only blowing air from the outside through the cabin, unless you have recirculate mode active. In that case, it’s just recycling the air in the cabin. It does not actually cool the air. Any sensation of coolness is simply because the air is moving, rather than still.