How to calculate power used by my microphone?
Microphones of that type are powered through the audio connection.  There is usually a resistor of some thousand(s) of ohms resistance.  That limits the amount of current that the microphone can draw.

Assuming 1000 ohms and 5V, your microphone could draw at most 5 milliamperes.   If it drew 5mA, though, you wouldn't get any audio out of it.

Compared to the current the camera itself will draw, the microphone doesn't matter.

Even without an external microphone, the internal microphone will draw a small amount of current.  Probably about as much as the the external microphone.

So, don't worry about the microphone.  It won't influence the recording time enough to matter.