How long can you run a 400w inverter on a 12v car battery?

Ecko gave a good answer. Based on experience, that 400 watt inverter is only going to give you about 389 watts of output power. Using a car battery is a bad idea. You can ruin the battery using it to power an inverter, A car battery was not designed for that kind of service. Forget the cranking amps rating on the battery. That current capacity is only good for a few seconds. The reserve capacity is the current that you have to consider. If you are not going to invest in a battery designed for inverters, then at least get a RV or Marine battery. Those would serve you much better than a car battery.  more