The guy who takes care of my cars at the service centre told me sometime back that in about half an hour around 1 litre would be consumed.
I asked him coz I would listen to my stereo with two amps in my parked car for 30-40 mins with the engine running! If I were to listen to my stereo without the engine running for long durations on battery power alone I would have fryed umpteen batteries by now!!
