An excellent starting guideline would be to hang the fixtures at eye level, which is approximately 5.5-6 ft. However, the ceiling's height and your fixture's size will affect the light's proper placement. An important point that you should remember is to ensure your wall lighting can't be reached by children.
Wall sconces are commonly installed in hallways. Long, dark corridors, however, can quickly start resembling runways if you add too many lights. A fixture every 8-10 feet will prove to be adequate. Of course, you may want less or more space in between every fixture depending on the size of your fixture and room. Staggering lights so they are not lining up across your hallway would appear to be a natural configuration.
Installing 1-2 wall sconces will serve as a room's decorative statement.