tl;dr: No one knows.
Long answer, Deoxys has been announced as the next EX boss. Despite this as well as the updates to the EX system by way of marked gyms and the ability to share passes no one knows for sure when the next round of passes are to be distributed. The last wave was sent 23 days ago. As for when to raid, it wouldn't hurt to start targeting EX gyms now.
I may be wrong here but I think we'll see some master updates on Deoxys before the passes go out. Most likely we'll see the catch rate bumped from 2% to 6% similar to Mewtwo. Hopefully we'll see a revision to its movepool before release, because it's remarkably poor as stands.
