Best answer to your friend's examiner would probably be "not very long"
I wonder. Probably better on a skill test to demonstrate that you've read and understood the emergency procedures section of the aeroplane's Flight Manual, don't you think?
Oh, and the battery duration has to be at least 30 minutes when only 75% of its rated capacity is available and where the 'action to be followed following illumination of the Low Bus Warning Volts warning light' is promptly completed ... drills that are set out in the same supplement to the AFM.