The word at Games Day yesterday was October for the new codex (which in GW terms, means November
I would imagine that with the release of the the codex will be a few of the items, like one or two units. Then, over the course of the next month, they will release the other units slowly. So...for example, with the 'dex could be a re-promotion of the new Chaos Space Marine box set, re-promo of the Berzerkers and Emperor's Children too. Then they'll release the Thousand Sons, two weeks later the Plague Marines, and then the Raptors along with the new Lord. But who knows? They could do it in any number of ways.