Usually one month. I'm not sure if they will redo the Chaos Daemon's one with just this 'Official Update'. If they did they would likely just pull out the 5 Seekers and add 10 Plague Bearers.
Also the new starter set will have new models, but there is a likely hood that both armies in the starter set will be getting new codex... as well as new models and Battle boxes. I am looking to likely pick up Chaos Space Marines, so hopefully I will have lots of new plastics and box sets.
_________________ I made the move to professionally painting 3 years ago. I paint a good mix between speed and quality. This keeps prices low, like a typical 2K 40k army is under $500.
Unless you play grey knights in which they will probably never release one for them.
There is no guarantee Games Workshop will release a Battleforce for every Codex, Grey Knights and Sisters of Battle are good examples. If I recall, the Dark Eldar BF was 5 - 6 months after the Codex was released, Blood Angels had to wait almost 9 months.
On the other hand, Necrons appeared to have a well planned and executed release schedule. Their Battleforce was 3 months after the codex, 3 months later 9 more models were on shelves. The latest batch was released soon after 6th edition along with a ton of Finecast and other Flyers. All of the units in the Necron codex were available within 9 months of the codex release. I hope this is a continuing trend with GW.