From his Reddit update 22 days ago:
If you look at the visual outline from the second update, I've finished everything for Part Two. My next task is to do a quick revision of Edgedancer to be turned in this week, and then do a revision of Part Two. I'm doing an unusual thing (for me) in revising each part after I finish it, then sending it to my team for continuity and editing. We discovered that a big slow-down in getting Word of Radiance ready was me waiting for the team to get back with increasingly-complex and detailed continuity notations.
This means when I finish the first draft of this book, it will actually be the second draft, which will speed up revisions a ton. (I should be able to move right into them, and do the third draft right away.)
The biggest challenge for the book will be making sure I don't go TOO long, as (like other Stormlight books) it's important to me that the book be read as a single volume, instead of as separate books published in a split-up way. (I can't prevent this in some markets, though.)