How much distance normally allowed above the mirror to fix the vanity light

Top of the mirror is usually around 6'-4, but it depends on the mirror size and what your preference is. I'm tall so I like it up higher. How high is the ceiling? Around 7 feet should be good for the light.

How much distance normally allowed above the mirror to fix the vanity light
From nothing to several inches. Depends on the fixture and your tastes.

There is NO set number nor is there a rule of thumb.

Mirror can be cut, and fixture(s) can be on mirror also.