This article from Paul Graham is a must read for anyone doing any kind of maker work (like writer or programmer).  Being aware of how much time you need to get productive work done is very important.  Once you’ve figured that out, trying to make that fit into most schedules is a nightmare (but that’s for another post).  Here’s an excerpt from the essay:

“…programmers and writers… generally prefer to use time in units of half a day at least. You can’t write or program well in units of an hour. That’s barely enough time to get started.”
Paul Graham, http://www.paulgraham.com/makersschedule.html