The short answer is sconces should be between 65″ to 70″ above the finished floor. Sconces above mirrors in bathrooms should be about 78″ above the floor.
That being said, the mounting height depends on the sconce, the height of the ceiling and what you want the sconce to do. Sconces can be used as task lights, like in a bathroom, they can be used as decorative elements or they can be used to provide indirect light, or a combination of all the above. Sconces can be short and compact, tall and wide or anything in between. Ceilings can be low or high. All of these factors determine how high to mount a sconce.
Before cutting a junction box into a wall I recommend cutting out a template and holding it against the wall to see how it looks. Bear in mind that sometimes the electrical connection on the back of the sconce is not in the middle of the sconce so the junction box might have to be higher or lower to maintain the finished height of the sconce. Also make sure of the dimensions of the back plate of the sconce. Some sconces have narrow back plates and will not fit over a regular junction box. You need a switch box for these sconces.
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