You have to be within range of the fob, just like for unlocking the car. You have to have "enough" charge in the traction battery. The temperature setting on the climate control has to be below the temperature inside the car.
It's real AC, the AC compressor is electrically powered so it doesn't need to have the ICE running to run the AC system. FWIW- your power steering pump, water pump and brake booster are also electrically operated. And, no belts (not even a timing belt) at all in the Gen III Prius!
I've only used it once, by accident. The range seems to be at least 15 feet. I discovered this with the fob in my pocket and must have accidentally compressed the AC button one day when mowing the lawn. I only discovered this when I noticed the car in the garage making a fan type sound then was surprised when I opened the door to find the AC running. It took me a few puzzled seconds to figure out what must have happened. Luckily I had the PiP plugged in. Not sure how long the AC was on for.
I believe that it will only run for 15 minutes, or something like that. By design, it isn't going to drain the battery.
I believe I read something like 10 or 15min and it will shut off. Or I guess if it hits the temp that your set at I'd assume it would shut off then too
I like that feature and I know my wife will this summer so when she goes to leave the house it will be cooled down. My almost 2 year old son will be happier to get in the car lol.