I find I have about 3 core hours of work in me -- 3 hours where I can flat out concentrate and produce high quality content/product.
I do these 3 hours in the morning -- pretty much every day. I shut off the phone, close my email client, and usually either code or write.
After 3 hours of that kind of work my brain just won't take any more. It's pretty much burned out on "high focus" work for the rest of the day.
After that I have to do "lighter" work -- such as correspondence and planning. Some days I'll do 2-3 more hours of light work. And some days I just get my core 3 in and go play disc golf in the afternoon
If you count the reading and studying I do, I actually get a couple extra hours in per day, but this is stuff I would do even if I wasn't trying to make money with it.
So what's your typical work day like?