I have grown Cattleyas successfully (and well) in many different kinds of media. In vivo they grow mostly on trees, often with some litter around the roots, sometimes in litter usually on hillsides, and sometimes on rocky outgrowths. They do well mounted on bark slabs if you can adjust you watering habits to suit. For our convenience (not the plant's) we usually grow in pots. There is no best single medium. It depends on your growing conditions. The medium must support the plant properly, drain well and allow air around the roots, be stable (not decay) for a prolonged period of time, not allow build-up of toxic by-products, have reasonable moisture retention, be easy to handle and not be too costly. I am sure there are other characteristics and readers are free to add to the list.
In my 35+ years of growing orchids I have tried many things. I started with bark but found that it broke down (became mushy) too quickly for my tastes, and I didn't like the variability from batch to batch. I tried Turface, which is inorganic and doesn't break down but is too heavy. I saw some wonderful Cattleyas growing in charcoal in Florida so I tried that. It was OK but dried out too quickly (I was growing in a greenhouse in MA). I added sponge rock (large perlite) and that helped, but adding a little chopped sphagnum moss helped moisture retention. I settled on a mix of 6 parts sponge rock, 3 parts charcoal and one part chopped sphagnum moss, adding a little oyster shell. This mix, in plastic pots, worked very well for me until I moved to a condo and switched to growing under lights. With a lot of air movement and a lot of light I found that the plants dried out a little too fast. I have been trying coco chips (presoaked to get rid of salts) and am now using 6 parts coco chips, three parts sponge rock and one part charcoal, which I am using now for most of my plants. So far the plants are thriving and the mix has been holding up very well (better than bark, IMHO) in plastic pots. I do add a little oyster shell.
BTW, I have also grown in Aliflor (fairly good experience), straight sphagnum moss (breaks down too fast and hard to untangle from the roots when re-potting), glass fiber of several types (poor results) and lord knows what else. There is no best single medium.
When to repot: when the medium starts to break down or when the plant is too big for the pot.
When, for Cattleyas, in their life cycle? I suggest you consult the excellent article in the fourth quarter, 2004 Orchid Digest by William Rogerson, with this additional caveat: if the plant is ailing because the medium has gone bad repot it immediately, trying to disturb the roots as little as possible.
Martin
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