52" to 56" to the top of the shelf is typical, but it depends more on the height of the fireplace opening and the specifications laid out in the installation manual for the fireplace. If the mantel is wood, it may need to be anywhere from 12" to 18" above the opening to the bottom of the mantel depending on how deep it is. A non-combustible steel mantel can be 12" above the opening.

Again, refer to the owner's manual of the fireplace.