quite a while. my signum said it was averaging 0.1 gallons/hour on tickover. I guess this is pretty approximate, but assuming this, a 12 gallon tank would last 120 hours = 5 days.
However, as the car would not be moving there may be a good chance of it overheating beforehand.
fred, enlighten me, please!!!!!! why leave a car running in that cicumstance? if you want to drain the tank there are way easier methods.and they don't harm the ozone.