There are no "rules" in art, and most of what is called a "rule" (such as the "Rule of Thirds") are more or less guidelines which to follow can, however, be quite, quite helpful. A guideline to when to turn the camera by 90 degrees is: the subject to be photographed is oblong, so the frame should be, too. This applies to faces, for example. Which is why using the vertical orientation is often also called Portrait Mode, whereas landscapes, e.g., stretch out to the sides, so you'd prefer to keep the camera in the horizontal orientation, often also referred to as Landscape Mode.