We say a decade before each CCD session begins. So the kids get through an entire set of mysteries every month or so. In that setting, we say the opening prayers every week, but not the closing prayers. At home, if you are saying each decade spread out throughout the day, I would only say the opening prayers (Creed, Our Father 3 Haily Marys) once and only say the closing prayers (Hail Holy Queen, etc.) once. But if you can only say one decade a day, I would say the opening prayers each time, just because I think saying the Creed is so important. If you are going to join a group recitation, you should follow the group norms of course. However, after all that, I will finish by saying at the rosary is a private devotion and you may say it however you’d like.