While there is no set rule for the distance a showerhead should be away from the wall, you need to consider a comfortable standing position and the diameter of your shower head to determine the most appropriate clearance. Most wall mounted shower arms are between 250mm and 400mm long, with a drop designed to go straight down to suit the rain style. Ceiling droppers are usually aesthetically appealing when they are centred within the shower area.
Wall showers, where the showerhead is angled so the spray reaches outwards, can be on shorter arms because of the angle of the spray.