One of the devs on facebook responded "hopefully in a couple of weeks, depends on the feedback we get from public testers."

Assuming there arent stability issues or broken mechanics I'd expect before the 8th of December. Thats apparently the deadline for some game awards thing. And if there are no major issues it seems reasonable, along with a few balance tweaks probably.

But balance tweaks notwithstanding, I'd recommend expecting Mistlands to be harsher than previous biomes. Time to git gud, people.

Like 6 months with these devs

So far I think IG has been pretty quick to fix bugs (with that I mean the time in test branch). Small updates around one week, bigger... two-three weeks?
How long was mountain update and H&H in test-branch? As I remember it, it was like a couple of weeks.

Considering the game is almost ludicrously stable it could be fairly quickly. Seriously, the only times I've found out about bugs is usually AFTER they've been squashed and I'm left going "that was a thing?" while looking at the patch notes.

I expect there to be a lot of bugs. All trying to kill you. It's Mistlands after all

I'll give you a super rough estimate - If all goes as planned, we'll see the update released around Christmas. If not - one or two months into the next year. All based on speculation, but I doubt you'll get an official answer.

Realistically it depends on how many bugs players find during the public test, but I'd say the earliest we'll see it (assuming it's all 'finished' and there are no game-breaking bugs) is mid-late December.