I have this lovely orchid too. I repotted it some months ago as I grew it in sphagnum which started to go bad for some time. Yet I waited till I saw a new growth at the base. I used the mix of medium barks, charcoal chips, perlite and some chopped shag for the repotting. The new growth seems to enjoy the new medium and has grown ever since. So I guess you can repot yours when you notice new growths.