The battery heater comes on when the pack (not the air temp) reaches single digit Fahrenheit temps. It uses 300 watts, at least with the 24kwh pack. It will run until either the pack warms enough or the pack reaches, IIRC, 30% SOC. Then it shuts off and the car "hibernates."
As for running the heat for hours, or driving the car to get the SOC way down, I'd only worry about doing that in warm weather. An actual SOC of 85-90% should be fine in cool weather.
As for running the heat for hours, or driving the car to get the SOC way down, I'd only worry about doing that in warm weather. An actual SOC of 85-90% should be fine in cool weather.