As I understand it, 'rare' ships are those previously available for CPs and no longer in 'production'. About how often are they cycled- i.e., when will the current CP Exchange ships be discontinued and replaced?
It is rotated in a somewhat random fashion - the release dates are not publicly released. If they were, people would save up, buy a whole bunch at the last minute and attempt to capitalize at their sudden rarity. With this method you plan ahead or get something early in the new batch. The same procedure is followed with droids and vehicles.