The Queen's Council, led by L'Evallier, keeps an official record or diary of its activities but there is no question of this ever being published. Neither monarch nor Council members would stomach that. It's a pity because the diaries are very frank though it would be inevitable if publication was an option, that frankness would go. (The best reason for not publishing them is that Roherum on the Fairy News Network would almost certainly exaggerate them and there is a genuine feel this would add nothing to the cause of good journalism).
There is no 30, 50 or 100 year rules in the realm. If something is not to be published, it is either destroyed or buried in the royal archives. These go on for miles under the Palace and even with the best location spell at your disposal if the Queen and her Council don't want you finding something, you won't.
Eileen wrote her own version and thoughts on various Kingdom historical incidents. These were published but only because of her royal status. Her books are safely buried in the Palace Library. Those who want to read them have to give their name to the stern Librarian and, surprise surprise, it puts people off. The Queen is very good at subtle censorship.