If you put in five or six hours of focused, high-quality work in a day (i.e. not spending most of it attending meetings, checking your e-mail, rearranging icons on your desktop or whatever), you're doing about as much as can be sustainably done; charge for that day with a clear conscience.

If you spend a couple of hours trying to concentrate before realizing you didn't really get anything done, you're just messing around and you need a break, take the day off and don't charge for it.