Backers get the book this summer; the book hits bookstores this fall. So a few months.
Collaborative writing is an interesting experience. I've written both nonfiction and fiction with my partner and coauthor Eve Rickert, and the experience is very different in each case.
For nonfiction, we start with a content outline that lists the subjects we want to write about. In the morning, we each go to the content outline, pick something we want to write, and start writing. Then over lunch we talk about what we've done, and in the afternoon we read, edit, rewrite, and/or leave comments on each others' writing.
That works really well, and the result is a book that melds our 'voices' quite well.
With fiction, I write every word. Eve and I talk about plot and characters (Eve does a lot of the plot; I create many of the characters); we do a timeline that spells out the plot; and then I write. Eve reads what I've written and makes suggestions, and I rewrite.
The 'voice' for our fiction is very distinctive; it's got quite a lot of Terry Pratchett or Douglas Adams in the sense that it's loopy, over-the-top humor. That kind of writing doesn't seem to work well if two people try to do it together, which is why with the fiction I do the actual writing.