2011 Leaf (resistance heat).
I have seen the heater use as much as 4.5 or 5 kW. When the AC button is turned on, the energy use can rise another .5 to 1 kW. I don't know if this is because it increases the power to the heater or because the compressor actually runs at the same time to defrost.
Actual power draw depends on a number of factors. The temperature inside the car, the temperature outside, the temperature of the heating coolant loop, AC button, and the fan speed all play a factor. I've noticed that selecting defrost tends to increase the power consumption. Likewise, higher fan speeds will call for more heating power.
As the car and the heating liquid heats up, the power consumption of the heater will begin to drop. It tends to equalize around 1.5 kW in 20-30F temps.
I don't believe Leafs after 2012 have a heating liquid loop. I believe they have a direct PTC heater. This may cause the behavior to change.
Finally, when the car is plugged in, it will not exceed the input power from the charger. On L1, this is around 1.5kW. L2 is 3.3 kW or 6.6 kW depending on the on board charger. Heat while plugged in only works if the car has a climate control temp timer enabled and the car is started by holding the brake while pushing the power button or the climate timer triggers.
Preheating the car while plugged in has been incredibly helpful. You can preheat not only the car but also the heating coolant, meaning that the battery never sees the high initial power draw.
Go to the energy info screen and play around. You can see the real-time power output of your car.