No code, just common sense. Leave enough room to:

- clear a tall faucet,
- show any pretty backsplash tile,
- allow for a possible future vessel sink,
- not have the door knock over toothbrush holder / tumbler / soap

But not so much space that:

- the bottom of the mirror is too high for short people,
- a light placed above would be too far away or too close to the ceiling.

I think 6-8" between countertop and bottom of mirror would satisfy most family members. If the mirror is too low you get a lot of splashed water on it.