About 4' up off the floor is standard but not hard and fast. We will move the line up or down on a job depending on the height and location of wall wsitches, outlets and such. If the mirror is in the way it can be padded out from behind with wood so the bottom clears the tile. If it is part of a medicine chest it should be installed first then the tile cut to it. I have seen plain mirrors mounted to the wall and tile extended up and over to make it look like a picture frame.

thanks so much, 48" is about where I was thinking the tile should go and I will pursue looking at framing with tile around a plate mirror so the mirror doesn't sit too high. appreciate your help