It depends on the motor and the load. A lightly loaded motor (small and/or shallow pitched prop) will draw much less current than one with too much prop. With brushless you can only go off what numbers the advertising claims since the motor is only half of the deal. The ESC is the part that directs the current to the motor coils. Nothing happens without it. And something to tell it what to do.
With a brushed motor, life is simple - you connect the battery and measure the current.
In the brushless world, you need an ESC with enough reserve on paper and something to control it with. Then you can hope that everything works when it is hooked up, and there is the chance to find out what it does actually take, but even when the average current is known, there is a huge amount of de-rating to do to take care of start-up loads and other transients.