Time is a shape-shifter. A piece you think will only take an hour (or a week) to write takes five. Something you put off, and put off, and put off, because you think it will be a huge undertaking, goes quickly and easily once you get to it.
The number of hours per day may be set, but how you experience them, whether they fly by or drag on forever, whether you get a lot written or very little, can’t be predetermined. Time management is a living, moment-to-moment, interactive experience.
Whatever writing schedule you set, know that it’s merely an estimate and be ready to go with the creative flow.