I can prepare a class in about the same amount of time it takes to teach that same class. In other words, I can take an hour and 15 minutes to prepare a class of equal length.
This excludes the extra time it takes me to read a lengthy novel, for example. If we are reading 60 pages for a particular day, then I would need an hour to read those pages, and maybe another half an hour to 45 minutes to prepare the actual class.
Sometimes, preparation is quicker because I have materials from previous semesters that I can use. Sometimes, it is much slower because the material is all new. Prep can be broken down into two aspects: knowing the material and planning the actual class.
I can grade x number of papers in an hour. Maybe 1 graduate paper, 2-3 undergraduate papers, 4-6 short compositions, etc...
So finitude ought to be possible with teaching as well as with research. If I teach five hours a week, I can spend five hours preparing and an average of two hours grading. That is twelve hours. Add meetings with students and other extras, like answering emails from them, that is about 15, if I don't have to read novels.
If I were better organized I am sure I could spend less time and be a better teacher than I am now. For example, I often just reprepare a class instead of using perfectly fine material I once prepared, simply because I cannot find it.
I am writing about this because I think that teaching is the missing link in the management of scholarly writing. I hear people say they cannot get other things done because they are teaching, or because the semester is too busy... Even I fall into this trap sometimes. If you treat teaching as a finite activity, requiring a certain number of hours, and you schedule those hours, then you will be able to see what time you might have for research. For example, if you are teaching 3 rather than 2 courses, then my twenty hours might be 30.