I couldn't find any definitive posts, but from search results for "900 pages" sorted in ascending order, it looks like the rule to remake threads only at 900+ pages was settled on sometime in late 2011, as there were cases in early-mid 2011 of people remaking some of the bigger/more popular threads much earlier (sometimes as early as 600 pages).
The closest posts I could find to being The Official Word are this one and this one and this one. (Atwood and Hekomi were moderators at the time.) Unfortunately these are kinda buried and I really wouldn't want to link to the second one on the front page of our makeshift official Rule Index lol
Other posts I found:
Forum/viewtopic.php?f=39&t=578644&p=21162076#p21162076
(I can't find the original post)
Forum/viewtopic.php?f=6&t=463157&p=10586004#p10586004
(In addition to being a long-time artist, Vampiric is technically also a long-time mod)
Forum/viewtopic.php?f=108&t=660821&p=15953762#p15953762
I also found some talk about public vs private threads (public threads can be remade by anyone, private threads cannot) but I have not seen any official mention of that in either my memories or in search results. Might just be a user-/community-defined thing.
TL;DR:
1. New threads should be made soon after the current thread reaches 900 pages.
2. New threads should not be made until the current thread reaches 900 pages. Threads cannot be remade in advance.