This is a fair and honest summary by Baron Schwartz of what it's really like. I wrote a book (and proof-read it better than this post) in 2005 and probably won't get a chance to do it again ("at least not with this wife!" says my dearest).
What a well-established publisher brought to the table for me was a guaranteed outlet for the book, more marketing than I would have had the energy or time to do, and a US$6K advance. They also provided high-quality images, indexing and copy-editing.
There's a nice progress chart of my work shown above and here with the total word count and words/day over time. If I were ever to do it again, I would estimate an overall rate of 500 words/day requiring at least 200 7-hour sessions, so 1400 hours. For comparison, my 120-page doctoral thesis took about 4 months of 6 day weeks with 10 to 12 hours/day, so about 1000 hours.