How do I calculate the discharge time of a battery with a step-down converter?
You can use:
293.76Wh / (12V * 7A)

Or:
293.76Wh / (14.4V * 7A*(12V/14.4))

Equals to:
293.76Wh / (14.4V * 5.83A)

In any case, you need to take current and voltage from the same part of the circuit. My first option is if power is measured near the motor. Second-third is if power is measured near the battery. Result is the same. Also it is a good idea to account for battery inefficiency at high current, it could be about 20% loss. And converter loss. Also could be about 20%.