Brian makes good points - lower level courses take less time to prepare for than upper division courses, and there is an economy of scale - teaching multiple sections of the same course greatly reduces the overhead of preparation.
That said, the first time a lecturer teaches a course, the workload is very high - depending on the course content, between 1-3 hours of prep for each hour of lecture (for the first section) is normal (this is an average over a whole semester). So if you had 20 contact hours per week, and that was (for example) two sections each of two new courses, then your prep workload might be something like:
- Course 1, Section 1 - 5 contact, 5-15 prep, 1 grading
- Course 1, Section 2 - 5 contact, 0 prep, 1 grading
- Course 2, Section 1 - 5 contact, 5-15 prep, 1 grading
- Course 2, Section 2 - 5 contact, 0 prep, 1 grading
This would work out to 34-54 hours per week for the first semester. In subsequent semesters, the prep time decreases greatly. You might need a minimum of 2 hours per week total using the example above, and if you spend more, that time can go into improving the quality of the lectures. So maybe you would spend 30-40 hours per week in subsequent semesters.
In short, the workload for the first semester is very high, but as you continue to teach the same courses, it decreases a lot. In my personal experience, after several semesters of teaching the same course, I can almost teach it from memory - so my prep time instead becomes "improvement" time. In the same respect, preparing one course makes preparing for other courses easier - you learn how to organize material more effectively, you find ways to re-use content and learning materials, and you develop teaching patterns that can be applied to multiple classes.
For all of these reasons, teaching is not really a profession that can be easily characterized by weekly workload. Some weeks you might put in 50-60 hours, some weeks maybe it would be only 20. But in general, for a full-time position the average should work out to somewhere around 40 hours per week.