GW is kinda notorious for not having set systematic operations like that. When 6th edition came out, all the old armybooks got scrapped and a small, magazine-sized supplement was released to hold over players until their armybook was released, for example. 7th and 8th saw a slow drip of updated armybooks, with some armies stuck in an older edition book while other armies had updated rules. The Bretonnians, for example, /still/ have a 6th edition book, two editions late.
For 9th, expect to see armybooks updated one at a time every 2-4 months if they keep up their old ways. It's hard to say with rumors of model lines/whole factions changing drastically.
It's safe to assume we won't see a new book for every single army on day 1, though. Bet your bottom dollar on that.