This is somewhat different but, back when I did consulting work that often involved writing in some form, we tended to think in terms of the work that we could do in a "production day" of about 4 hours. The thinking was that, on a given day, we'd spend about half on project-based work and about half on the phone with clients and others, doing miscellaneous research and "research," internal discussions, and various administrivia. Sometimes we'd buckle down more to get something done and other times we were at conferences and the like--but we found this a good rule-of-thumb.