On average, a car uses between 1/5 to 1/7 of a gallon of gas per hour when it is idling. There are different types of engines requiring differing amounts of fuel necessary to power the engine.
A common myth has many believing that turning cars on and off at long stops decreases engine efficiency. After the introduction of fuel-injected engines, the idea that held this belief together fell apart. A fuel-injected engine produces the correct fuel mixture to ensure proper combustion, protecting the engine and dispelling this notion. Today, some car manufacturers are installing systems in their cars that turn off after coming to a complete stop and then restarting when leaving. For example, hybrids turn off their engines while at a standstill to improve gas mileage.