I don't have any particular preference. I have to go with whether or not I'm working on a project at any given time. (I'm self-employed.) If I have a slow period or between-projects type of situation, I'll write 20 hours a day (and then use the HP forums and Answers sections as my way of taking breaks, or else get ideas for future writing). That doesn't happen all that often. Right now things are kind of slow, and I'm planning to have not only a no-work-hours for a few days, but a few days of no-writing-hours-at-all either.
One way or another, and having said all that; it all amounts to my writing when time, mood, and energy level permit (or else when someone's deadline dictates it).