As I've started my new job at Universidad San Francisco de Quito, I got 3 courses assigned to teach in the Spring semester. Three new courses to teach means three new courses to prepare - which initially sounded like a ton of work to me.
For reference, the first lecture I ever taught took me roughly 30 hours to prepare. No way I can spend 15 hours per hour of class when I'm teaching 9 hours of class per week...
To figure out how much time I should schedule, and get a realistic idea of what to expect next semester, I turned to the Biggest Oracle of our World aka the internet.
First of all, I stumbled upon this great resource from University of Chicago on preparing to teach. This document takes you step-by-step through the process of preparing a new course: from the moment you put your syllabus together to the actual delivery of your class.
Then, I found that the American Faculty Association discusses the topic of class preparation time very directly. They give 2 to 4 hours of preparation time as a rule of thumb. My planning is happy with getting a real estimate of the required time.
I also learned (not really to my surprise) that, as with everything, there is a law of diminishing returns for class preparation time. The more time you plan to spend preparing, the more time it will take up. We know that from PhD research too. The guideline from this resource is "Two hours for new lectures and half an hour for lectures you've given before is a good guideline". The authors of the article suggest a "mean and lean" approach to preparing for your class - and I am planning to follow this approach.
Finally, I turned to Twitter to get advice from more experienced lecturers. Many times, 4 hours of preparation for a new class is coming up. The Storify of this discussion is here at the bottom of this post.
In conclusion: I'm scheduling 4 hours of preparation per class. As suggested in the "mean and lean" approach, I am subdividing that into the following activities:
45 minutes to read the class material
45 minutes to distill all important information
15 minutes to outline the class hour
45 minutes to construct the argument around the "what if" question
45 minutes to prepare an example and possible homework
45 minutes right before class to make some slides with figures that are complicated to draw on the blackboard, or pictures from practice, and to revise all material.