So I watched this for about a year and usually once Vrelnir pushes the changelog and their own work, the release comes about 2-3 days later. Vrelnir's push was yesterday.
However, that's for 1.5~ month of update, this one took 3 months. The 6~ month pregnancy update took multiple weeks and the larger the update, the more complicated debugging gets (its not just linear)
So I think earliest possible release, humanly possible, would be this night, 3AM~, about 16 hours from now.
But this is exceedingly unlikely. First realistic time is about 40 hours from now, in my opinion.
Most likely release time, I would guess, would be this friday. This would mean they have about 6 days of fixes, I'd expect them to do it early that day so they can do hotfixes that day and spend the weekend celebrating freedom. People tend to have more plans for weekends. Thursday is possible if it goes very well, it could also be they don't have many weekend plans and intend to release sunday and spend that day babying it. Updating does usually mean babying it for a while.
It's possible for it to take another week, but I'd consider more than that exceedingly unlikely. I'd gamble around friday.
Also, 4.2.1 is already in the repo, but I won't post the notes as it's pretty irrelevant (just fixes, as much as the lovely people deserve credit, it's not an interesting read)