is right after you finish the previous one. The same day if possible. Take advantage of that momentum. My daughter's music teacher said that right after an audition is a good time to do a lot of practicing. Why? Because you have peaked for that audition, so the practice will be in that higher zone and thus be more effective. With writing, you can take advantage of the fact that you were just now writing very well, in finished prose rather than in rough notes, in order to complete the last thing you wrote. Mentally, you will be riding that wave of elation rather than experiencing let-down.
I recently finished an article and two chapters, so I am beginning with chapter that has intimidated me in the past and that I have to conclude by February. If I finish this, I will be 5/8 done with the book, more or less.
If you want to take a short break of a day or two, then, don't do it between chapters, but in the middle of one of them.