For the most part, having more money and more fame make your life harder. It just does. I've seen it happen with people. You know, it's so hard to stay normal. It's so hard to stay happy. It's hard to remember why you were doing what you did in the first place.
At the very end of a book I can manage to work for longer stretches, but mostly, making stuff up for three hours, that's enough. I can't do any more. At the end of the day I might tinker with my morning's work and maybe write some again. But I think three hours is fine.