I read this article this morning that quoted research saying that people only have the capacity for a few hours of really intensive concentration per day -- after that, attention and quality tails off considerably.
I kind of believe that. I've always found that I have around 3 or 4 "good" writing hours per day. I can write longer, and often do when I have to, but that additional time is usually not as focused or productive and the result isn't as good.
This week I'm really stretching that. But that other project did not fall through after all, so I'm totally on the hook.