The best time to repot: We repot in late spring and early summer before the onset of vigorous new growth. Since roots that have been growing exposed to the air for any length of time will most likely die when packed beneath the bark surface, try to repot just as new growth is beginning to appear. If you waited too long and your plant is already well into its new season, hold off until after your orchid has bloomed, otherwise you will lose that growth’s flowers and you’ll run the risk of killing the plant’s entire new root system.
Repotting should be done every year-and-a-half to two years, before the potting medium begins to break down into peat and loam. Waiting longer than two years allows the broken-down medium to retain too much moisture, keeps the roots from receiving any air circulation, and will contribute to the onset of root rot.
Here is a link to an article on repotting a cattleya: How to repot a Cattleya Orchid - step by step.