As Bumber said, it's an extra long wait in between the last release of this development arc and the first one in the (first) Myth & Magic arc. Toady has aimed at keeping these release free period down to about a year, but at the same time he's managed to keep game saves backwards compatible (with new feature loss) for quite some time.
This time he's going to do significant structural changes that are going to break save game compatibility, and he'll probably cram several of those in at the same time. Then he's going to implement basic myth and magic, as well as mostly restore the current functionality using the new structures.
As can be guessed, this takes extra time. How long isn't known (obviously), but it's probably not going to be less than two years and hopefully not more than four.
I believe the start of the Big Wait will happen a lot later than in a few weeks. The development page contains a lot of candidates for development in this arc, and while he's stated clearly that not all of them will be implemented now, there's several month's worth of development material there, and once the actual development is done, there's usually one or two bug fixing releases at the end to leave the game in a playable state during the Big Wait. I've seen someone guess at October, and that seems like the right ballpark to me, give or take a couple of months.