It's advisable to turn your car off when stopping for extended period.

Reason being, by letting the car idle ,especially with the AC on, you're basically aiding in the car wearing without racking up any miles. With the cylinders firing, you are using several electrical components, putting some small amount of stress on the belts, etc.

Stopping your car for a short, 2-3 minutes is fine; anything over 10 minutes would be advisable to turn your car off.