In order to comply with National Fire Protection Association guidelines, your mantel and any other combustible material must be at least 6 inches above your fireplace opening. If your mantel projects outward more than 1.5 inches beyond your fireplace opening, the minimum height is raised to 12 inches.
Depending on the size of your fireplace, the size of the mantel and the height of your wall, you may want to hang it significantly higher than the required 6- or 12-inch safety requirement. For a mantel that is designed to be a focal point, you will want it to be at eye level, about 5 to 6 feet above the ground. If you intend to place decorative objects on the mantel, you will want to lower the mantel so that the objects are at eye-level.
You should comply with safety regulations and meet the minimum requirements to avoid the risk of fire. After that, choose the height based on the scale of the room, the intended purpose of your mantel and your design preferences.