When I tested it on the PTS, the container was a guaranteed drop every boss, including those in private instances (group dungeons), so there is no need to compete or camp. No such thing as a cooldown. Each container was guaranteed either a motif page or a recipe. This was three months ago, but nobody has been able to test it since, so this is all that anyone would be able to tell you.
Frankly, I don't get why people are so worked up about this. Just try it out on the 13th. It's not like knowing these details ahead of time will change anything.