This is not an unusual question, but quite often one that is overlooked when decorating the home. Keep in mind, the purpose of the mirror is to reflect light, so be sure to take into account the wall that you are placing it on and the direction of the sun at all times of the day.
Typically it is recommended to place the center of the mirror about 60-62 inches from the floor, but of course it depends upon the location and type of mirror. Generally, this height would be if you are trying to place the mirror above furniture that is under 30 inches tall or if it is to be the only item on the wall. That way it will be in the line of sight for the average-height person.
Mirrors placed in the bedroom above the headboard and sofas should have at least a 5″ gap between it and the base of the mirror, with a maximum of a 10″ gap depending upon the height of the headboard or sofa. Mirrors that will hang over furniture that is greater than 30 inches are placed based upon a 3 to 10 inch gap from the base of the mirror. This also includes mantles as well… rule of thumb, the taller the item, the smaller the gap.