This is completely subjective and depends on many things, including the height of the person using the mirror, the style and shape of both the mirror and the sconces, as well as the ceiling height of the room. Some photos would help.

We found most suggestions for side sconces were about face level, so could vary by your height. (Not sure if you mean side sconces or lighting above mirror). We measured it all out for our contractors to get exactly what we wanted. Not sure of mirror height but we have 4" granite backsplash and bottom of mirror is maybe 2 inches above that.

This is very helpful!

No standards. It is more a feeling of balance. Some mirrors go to the counter, some start above a back-splash and some are hung a bit higher. If it is a children's bathroom lower is better so they can see. I am guessing the sconce lights are 27" - 36" above the counter. Again, balance, style, up or down lights all have a lot to do with this. On the lights make sure they give good work light but don't blind you. Sometimes because of height differences what is comfortable for one person is not for another.