I'm not sure how far in advance they book out - at least a year. But as far as how far in advance you should book, that depends on how you will be booking and when you go. Peak seasons like Christmas, Spring Break, and even parts of summer and winter tend to be busy, so you should book more in advance for those times. We visited in September which is a quieter time of year, and we would definitely go that time of year again.
If you want to look into renting DVC points for your stay, you should book 7-11 months in advance. We were able to secure ours at 10 months out.