Up to the point when sales begin,
DVC owns the entire room inventory, so it's really up to them to decide how many they release to be booked by the general public, and when they're available. Short answer: it's at the whim of DVC management, so who knows.
Does anyone remember from past DVC resort openings if there's typically advance notice of when rooms will become available to the general public? Or do they just appear in the system?