What's a realistic workload per blackboard hour for teachers during the semester? There is this excellent thread about teaching workloads across the world on Academia SE: Is 100 hours per year of teaching a heavy load for a lecturer?

I would now like to continue this discussion with respect to the actual workload teaching staff face. I know that teaching is not only blackboard/classroom hours, but of hours of preparation, grading, examinations etc. Since exams and grading usually happen during the semester break at universities, I am more interested in the workload teaching staff face during the semester. Of course, creating a new class from scratch consumes an enormous amount of work, so this is not what I would like to use as baseline. I'd much rather go for your typical class of well-established contents, maybe undergrad stuff, for which only occasionally a minor update of materials is required - and not a complete makeover every year. To formulate the question precisely: What is your actual workload during the semester per blackboard hour in classes that belong to your established portfolio?
In my opinion this is hugely course-dependent, and standards vary a lot between universities and disciplines.

If your task in the class is really almost only lecturing (e.g., you have assistants that run any class projects etc., and who answer most of the "standard" student questions), and you already have everything prepared, your actual effort for the course may be close to the "blackboard time" - but even in such cases, you probably want to count in at least half an hour before each class to refresh your memory on what material you plan to cover in a specific unit.

If you have any other tasks at all in the course, "blackboard time" quickly becomes a very bad proxy for your actual effort. In my experience, course planning, answering student questions, office hours, organizing and/or holding supervision sessions, giving feedback on in-class exercises, dealing with exceptions, preparing and grading exams, or dealing with teaching infrastructure can take a large, and highly variable, amount of time. These factors are also virtually independent of how often you actually speak in front of the class. Not all classes have all of those activities, but most classes have at least a few of the above elements.

In the course I recently wrapped up, I would estimate that a maximum of 1/5 of my actual work was done "in class", while 4/5 was preparing, grading/giving feedback, and organizing. However, in other classes I have spent close to 70% or 80% of my course work time "in class". It really depends on the course, your role in it, and how effective you personally are. Unless you have good reasons to believe this will not be the case I would assume that your workload outside of class will at least be as high as in-class (i.e., assume that if you are teaching one full day a week you will spend at least another day dealing with the various paraphernalia of teaching, even if you have everything set up and prepared - if this is not the case, I personally calculate that I will spend at the minimum one full day to prepare a two-hour session from scratch).