No one knows, likely not even the devs :) timeframe? Also no idea, it was a few months between hearth and home and frost caves, but this is a whole new biome, so that means

1. Yagluths item will become an actual progression tool, so whatever new mechanic it opens needs to be worked out

2. Terrain/lighting/music

3.new mobs

4.new items and build pieces and food

5.new dungeon

6. New forsaken boss fight, with its own mechanism.animations etc. with forsaken power, and a redesign of spawn so that the new sacrificial stone fits

TLDR this is a know much bigger update then either of the last two , no one knows how long it will take, but there will also be a lot more too this update then what has been added previously :D