Annie Dillard, a prolific essayist, wrote about having a writing schedule, concluding that it's a net that traps that fleeting commodity, time. So I embark tomorrow (not today! Too busy today!!) upon an entirely scheduled writing week as an experiment. I think four hours a day, in the morning, let's say 8 to 12, is reasonable for writing and writing-related duties (writing in journal does not count; reading literary magazines does count); two hours a day for exercise, housework or yard work; two to six hours for paying work; & the rest open. I will let you know whether rigidly dedicated writing time turns out to be productive -- so many writers have said it is -- or if I can't make myself do it for seven straight days, and why.
I tend to start with unfinished material, tinkering and thinking, and within a few days get totally in gear, ready to draft new material.