How far in should water pipes be insulated on exterior walls?

If the area you live in is prone to freezing conditions, then you should not have pipes on exterior walls, they could easily freeze. First thing to do is to get them relocated, if that's not possible, make sure the wall itself is insulated, no drafts, no air coming in from outside, fill in holes, then run insulation the length of the pipe. if the pipe is coming straight through the exterior wall out into the room, best to wrap it minimum 4 feet from the wall.