To be quite honest, I can churn out a book every 4 months. I could have probably had another 4 books done by now, besides the two I churned out a year ago.
This is if things flow relatively well. The ideas have to continue to flow. You need time, and not to rush things. If one thing I find, if a person does write for 8 hrs a day, you perhaps end up with allot of burned ideas turning up on the page. But, not everyone is probably like this.
Rarely I do write for 8 hrs straight. Sometimes I will go at it for an hour or two straight. Get up, get a coffee or tea, or beer, then go at it for another hour or two. Breaks may last approximately 20 minutes, during some mental blocks.
4 hrs straight is a great writing stretch in a day. It does not mentally exhaust you for the next day, when you say "Oh drat, I have to start writing again today!". The energy levels are there, and your mind feels fresh like a bar of Irish Spring Soap..
Have a great day (no news on the book front), till next time....