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.
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.