You can't really calculate an accurate run time, because throttle usage varies, and running at less-than-full will extend run times somewhat. A crude estimate can be done by dividing the capacity of the battery by the full-throttle current of the motor.

mAh == milliAmpere-hours
A == Amps * 1000 == milliAmperes

Divide mAh by mA, you get hours, or the run time of the motor, roughly, and only if you run full throttle the whole time.