Historically speaking, they don’t tend to release much if any information on this type of thing–however invitations tend to be somewhat regular in pace.
There’s a need for constant replenishment to keep new people flowing through starting content, people get bored quickly and stop logging in (beta isn’t a free preview–there’s usually some serious bugs that people don’t like dealing with and content is rarely feature complete), and it maintains a certain interest level in what is not only a bug search, but also a marketing tool.