When to buy is pretty difficult to answer. Default rule of thumb is as early as possible, particularly if you are set on going on a specific itinerary and date. However, RC and most other cruise lines will adjust specific sailing prices up and down according to how sales are going. More often than not, the ship's analytics are pretty good and prices will slowly go up as the ship fills. Where you might see prices go down is if it's a relatively new itinerary that they don't have a lot of data on or if there was a change (eg. ship had to change itinerary due to departure/port of call issues like conflict or weather).

Another milestone to watch for is just past final payment date which is 75 days prior to the cruise for RC. After final payment, they kind of know who's committed to the cruise and how many cabin they still have to fill. If there are a lot of cabins left to fill there might be some discounting. But it's obviously a risk booking booking 75 days out regarding pricing, cabin availability, flights, etc.