Like most MMO betas these days, later this month invites will go out for specific play sessions -- meaning, there will only be certain windows in which to wrack up some playtime. According to the FAQ, every beta will have a different purpose and focus.
Initially, the betas will be held on weekends (similar to Guild Wars 2), and as time goes on, more events will be held with more participants. Pretty standard stuff.
If you haven't done so, go sign up for the beta and take your chances.
Your Beta Questions Answered [Elder Scrolls Online]
There won't be a public beta of the latest Halo: The Master Chief Collection update
7:00 PM on 02.10.2015