Idling an engine for as little as 10 seconds will use more fuel than stopping and restarting the vehicle. Stop-start systems that automatically shut down the engine at idle to save fuel are standard on all hybrid vehicles and are becoming more common on conventional vehicles. The fuel used when idling varies by the amount of heat/air used, accessories used (radio, lights, and fans), vehicle make and model, ambient conditions, and many other factors.
Estimates from CSRA based upon work by Argonne National Laboratory.
Argonne National Laboratory, Which Is Greener: Idle, or Stop and Restart? Comparing Fuel Use and Emissions for Short Passenger-Car Stops, Argonne National Laboratory, 2013.