Amount of time to leave the car running?

I heard somewhere that it was cheaper in gas (not emissions) to leave the car running for 15 minutes rather than turning it off and turning it back on (to go into the post office, for example). Does anyone know where to find the actual number of minutes for which this is true? I’m sure it depends on the car, but I’m looking for a ballpark estimate.

If you know the amount of emissions generated by starting a car versus leaving it running, that would be helpful also.