This is a tricky subject, as the advice given above cannot be spread out to all genera of orchids.
Some like to be repotted after blooming, others after new growth. My personal opinion is this....
New plants, repotted immediately. Even if it has a flower on it. We have a problem with heat down here, and medium can brake down rather quickly, or having the wrong media can lead to other problems if not removed.
That being said, I would offer the advice on knowing WHEN to repot. It goes like this....
There are two times when an orchid needs to be repotted. WHEN you WANT TO, or WHEN you HAVE TO.
When you want to for me is when I see new roots growing (mostly catts) and just as they are about 1.5-2" long, I repot.
When you have to is when something doesn't look right, even though there is no growth, you suspect it needs a change (rot, wrong media, wrong pot, etc,,,).
Others can pick up on this, as they may know more than I, but some oprchids HATE to be repotted.... %#$*^ psychopsis papilio