Good questions! My answer is: depends on the day. I get more done during breaks. 0 to 2 hours on normal days.
Really, during semesters of teaching, my writing is narrowed down. I often make lists of strategies, things I want to write, etc. then go to that list when I have time.
I take the William Stafford approach, by waking up early in the morning and heading to PT's or the office to get some writing done. I also write to my friend Kevin Rabas, around three letters a week. He sends me letters too, so it is a terrific way to write, reflect, correspond.
I get most of my writing done during winter break, or during summer vacations. I know a lot of my writer friends do well in winter, too, as one gets isolated, turns inward. Also, there is something about getting away that helps with creativity. I wrote/compiled My Secret Wars of 1984 during a vacation in 2011, and it should be my first full-length book. I've had the most published from that.
I can't write when it is late. Instead, when I can't sleep, I watch Netflix. I am more poetic, more productive in the morning.
This semester, I wrote eight flash fiction pieces with my fiction writing class. I polished two. Now it will be the basis of my first novel. I think I can pull it off by writing flash fiction, around the overall story arc.
It's a strictly-randomly planned routine.