This is one of those things that tends to change, but I think my answer is still correct: not more than 30 days in advance, and it probably won't be an exact 30 day window. They seem to open up a calendar month's worth of slots at a time.
There is often a thread watching for them to open up; try watching the dining board. I was finally able to get one for our Aug 3-8 trip after being shut out for both Thanksgiving and New Year's weeks. Good luck! We had a nice time there.