The first time I taught a class, it took me (on average) 6-8 hours per hour of class time to prep, not including exam writing, homework assigning, or grading, which is obviously unsustainable. As I got more experienced with the material (and with teaching), I could drop this down to 30-45 minute per hour of class time. The first time I taught a new class, I was panicked to think I'd be back to 6-8 hours, but teaching itself is a learned skill, and I find it takes a lot less time to prep even new material.
Less time, that is, than 6-8 hours. It still takes me a long time to prep new material. I like to do a good job, so I try not to skimp on the class prep. Before I start a new course, I dread the extra work. It seems like a huge mountain of additional stuff I don't have time for. That said, I find that once I am into it, it goes faster than I fear, and I enjoy it. I like learning new things, and I find that having to explain things to students helps me deepen my understanding, even of material that I know fairly well. When the course includes things I haven't really used since I was a student, I find that my much deeper knowledge base now makes me appreciate things I glossed over as a student. It is still a ton of work, though.
My department here at ProdigalU does a good job protecting TT folks pre-tenure. Most people get to keep the courses they start with for their whole run to tenure. Post tenure, our department tries to give people a minimum of 3 years with a new course. However, since life happens (with emergencies and sabbaticals and family leaves and everything else), sometimes people have to shift more often.
I think it is important to rotate instructors. Not just from a fairness perspective (since we all know that some courses are more work than others), but from a teaching perspective as well. I find that 5-6 years is the ideal time for me to have a course before swapping. The first 2 years, I am still getting a feel for the material and how the students respond to it. I'd say I am at peak teaching performance (for me, anyway) in years 3 and 4. By year 5, I find that I am getting a bit stale in the class. Definitely by year 6, I am ready to move on.