Complex as always.
1. It depends on how often you water. Soil is only for terrestrials. Bark holds moisture so usually once a week waterers use it and you need to repot every year or two. Sphagnum is really only good for babies because it can rot roots so easily.
2. Depends on the orchid. Some orchids want to have constantly damp roots and some want to dry out. Ones that want to dry out either need to be watered less or live in a fast draining media or even be mounted.
3. Then there are your conditions. In a greenhouse in a sunny world, the medium dries fast. In my wet and relatively dark conditions, nothing dries out. So I mount and use fast draining and hard to break down media.
This is why we can only advise from our own experience. No one has a definitive answer, only suggested basic guidelines.
So what kind of orchids are you potting?
