The average knowledge worker (I would call writers knowledge workers) only has six hours of productive time per day. That’s thirty hours a week, if you write five days a week; thirty-six if you write on Saturday, too; and six to twelve if you only have time to write on weekends.
You must rest. Your book will not be better if you spend ten or twelve hours a day on it. In fact, it will probably be worse.
If you want to write a masterpiece, you have only a few hours a day to do it. Use them wisely.
And once they’re over, get up and walk away.