Like most orchids, Dendrobiums should be repotted when their potting mix starts to break down or the growth cannot be contained within the current pot. Dendrobiums like to be potted tightly (Crowded or root bound) for the most part. Don't over pot. If you have a photo of the orchid you are considering repotting, you could post it and let the members here take a look and advise you. Good luck and welcome to OrchidTalk, John.