I'm aware this will be subject-dependent, so, I'm in English Lit.
I have three solid chapters of what will be a six-chapter book. These have all been through conference papers and similar, and I submitted a journal article that's the basis for part of one of them, a couple of months ago.
I also have another chapter that's drafted but not polished.
I have an idea of the final two chapters, but they need a lot of work and the shape of them will inevitably change quite a bit before they're ready.
I've spoken informally to a publisher and the editor of the publisher's series, and both were encouraging about the general topic. However, this will be my first book. It's skipped ahead of my PhD revisions, so the PhD book is considerably less publishable at this stage.
At what point would you submit a proposal?
I'm aware people submit proposals well before they've completed their books, but the people I know in RL published their PhDs first, so they had some credibility already. I'm credible in that I'm employed (coming to the end of a two-year teaching postdoc), but that is it!