As a general rule, Cattleyas should be repotted every two to three years unless they are in semi-hydroponic culture or some media that does not break down. In that case they should be repotted when the plant becomes too big for its container. When working with the Cattleya Alliance you have two questions to answer: "When do I repot?" and "Do I want to divide the plant?" This article will address when to repot and a future article will address dividing the plant.
Cattleya are sympodial orchids, orchids that grow sideways along a stem that looks like a rhizome. Roots will grow out of the stem as the pseudobulb forms and grows upward. With some Catts the plants will literally grow out of a pot within a year, others will grow in a more upward fashion with little room between pseudobulbs on the rhizome. It will be quite obvious when a plant is creeping out of its pot that it requires repotting; the more mannerly types will only grow out of the pot when there is nowhere else to go. The middle of the plant will start to die off and the new growth will not be as large as the previous year's growth. Generally flower count will also deteriorate. You are indeed a good grower with a happy plant if you have gotten a specimen to this point!
As to when to repot. Wait for the plant to finish flowering and start new root growth. This is especially important in Cattleya Alliance plants as they are so hybridized. This will generally occur anywhere from April to June in most plants. Remove the plant from its container and wash off all the roots being sure to eliminate any media attached to them. Hold the plant and roots up to the new container and determine what length will be about three-quarters into the new pot and then clip all the roots that will be going in the pot to encourage new growth. Be sure that you are not trying to push air roots into the pot, they will rot and die if put in medium, leave them to grow outside the new pot.
Any medium you use for repotting your orchids should be soaked for approximately 24 hours prior to use. It needs to have enough time to absorb water so that the newly planted roots are not desiccated. Prepare the pot with drainage material at the bottom of the pot, and place a portion of medium over it at the bottom. Place your plant into the new pot and then carefully pour the medium around the roots. Tamp down carefully with your fingers, or a bamboo stake. I like to put bamboo stakes or shish kabob skewers on two or three sides of a plant to hold it in place until the plant has taken hold and becomes established in the pot.
Put the plant in a warmer and lower light position than it had been growing in before to encourage it to grow new roots and then move to its normal position in about a month.