Well that is a loaded questions - Short answer is all day. Long answer it depends.
Horse showing is a great game of patience. You have a schedule (I use that term very loosely), but the timing of said schedule is dependent on many factors. Some of these factors can be controlled by the gate person and the judge. Yet other not so much. The biggest factor is the number of riders entered and how fast their trainer can get from one ring to the next. Often trainers have riders in multiple rings and you need the trainer to compete at your ring. If something happens like a fall that takes extra time to get the horse and rider out and the next rider into the ring.
So the brief answer is it will be an all day event most likely from early in the morning to late afternoon. You can get lucky and be done in a couple hours, which has happened to me but not very often.
Enjoy the day and the experience!
