Ever have a sabbatical where you didn't get very much done? Very much, that is, in proportion to the time you had off? A semester's sabbatical would be from mid-December to mid-August, or mid-May to mid-January. A year-long one would be from May to mid August of the next year. That's seven months or even fifteen. But probably the actual work that got done was not proportional to that time. You may even get more done during a semester of teaching.
I know exactly why that happens: an excess of unstructured time. It would be like having your office be a huge warehouse. I don't know about you, I'd rather have my office in large closet than in an open field. An overabundance of space doesn't really make an office better. Having too much time is damaging in an analogous way. It can be extremely demoralizing to have a lot of time and not be productive. You'll have empty time and be bored, but still won't be getting done what you want to get done. Other obligations will invade your working time.
So what you want to do is design the time of a sabbatical to make it more like a regular semester of teaching. Design your week very deliberately. You can follow my model of using Sunday for planning, working fairly hard Monday through Thursday, and doing lighter work on Friday and even lighter work on Saturday. Or some other design more to your own personal style. The important thing is to write it down and follow it.