Both Halo 3 and Halo Reach had betas four months prior to the release date. I'm not saying that will be the same for this, because of a different company and a new trilogy, but that is my educated guess.
Because it is set to release 'Holiday 2012' I'm assuming we will get a beta around the end of summer.
Both games showed in-game footage nine months before release.
First In-game footage: 3-4 months Beta: 7-8 months