I know that I have roughly 5-6 hours worth of productive programming time in any given day. If I had a job where I was required to be on-site for eight hours a day, I'd get roughly the same amount of work done. Maybe even less, since I'd practice sitting at my work desk not doing things for 2-3 hours per day. I'm a huge fan of the whole "notice you're not accomplishing much -> go home" plan.