Hmmmm..I repot when I'm switching something over, for example, from pot to mount, or when it's necessary.
I buy a lot of seedlings, and I usually get them with really bad roots or very small. I pot those until they get good, solid roots and then I decide whether I'm gonna keep the plant in the pot or mount it.
ETA: The ONLY golden rule I have regarding orchid growing is that I repot everything that I buy. When I mean everything, I MEAN everything. I don't care if it's recently potted, recently divided, etc, I repot. Why, because 1) I grow in 100% hydroton, and chances are the plant is not in 100% hydroton. and 2) this prevents me from letting something loose in my collection. Every orchid that enters my collection goes through a repotting, cleaning, sterilizing, nipping dead roots, etc. process. I'm really picky about this.
