The remaining time, in my experience, is to coordinate meetings with management.

Managers at my company have meetings almost all day. I, however, only have a few meetings per week. They can do meetings for 8 hours just fine, but I can't code for 8 hours. I could go home after 4 hours and be just as productive per week, but I'd miss potential meeting hours.

And naturally, they are the ones whom we have to cater to when this kind of divide comes up.

http://www.paulgraham.com/makersschedule.html

Totally. I only have about 4-6 of focused mental energy before I'm just taking up space and feeling sleepy or antsy. So these days I head home after six hours in the office. Honestly I hope to eventually get to the point here that if I need a break, change of pace, or am just "done" I can do what makes sense... Take a nap, work from home, whatever.

Ha, I'm the same way. 4-5 hours, I'd say. I was recently very upfront with my employer about this and I managed to finagle myself a deal where they're halving my salary and I'm going down to four hours per day. They said they would have never hired me part time like that but luckily for me they don't want to lose me. May be worth a shot if you don't mind making less money.