How much time do you spend prepping lecture materials?

[Kron3007]

Where you have to change is in not being satisfied with what you have prepared.  The preparation will necessarily take a lot of time the first time.  (Or you run out of time the first time around, and some classes are conducted on a wing and a prayer, so you prepare them more thoroughly the second time around.)  Once the material is there, unless it goes badly wrong, leave it the way it is. Put it away neatly after class, ready to take out again next year.  Every year you'll find some stuff that didn't work, so you change that — in as efficient a way as possible.  That way the course gets better every year, incrementally.  But continual redoing to make marginal improvements, or just to make it "fresh" — no no no.  That's just anxious busywork, or maybe it's a way of avoiding your research obligations.  But it's a sure way to a) not have published enough to get tenure b) insure that you will have no life outside of work.  Stop it.

This.  I tell my advisees that teaching is like a gas--it will expand to fill all available time.  Even the best class can be improved upon.  You need to decide where to draw the line so you don't spend all your time on your teaching prep.  The goal should be "good enough," not perfection.

I accept the intent of this, but if you are in a rapidly developing field (ie. gene editing) you better be revamping the course or it will be outdated in no time.

For me, I spent at least a few hours per hour of lecture (probably more) the first time I taught my current courses.  Now that I have done them a few times, I really just spend maybe half an hour per hour going over the slides looking for typos and revamping the material unless it is a lecture that needs significant updating or changes.  For my lab, it is about the same, but takes more time even when it is well developed.  I have been modifying the lab a lot more than the lectures though, keeping things that work well and replacing those that dont.  I think in a couple more years the course will be where I want it.

For courses that I was not keeping longer term (the first few years I taught a variety of courses on a one time basis), I put in less time as it was not a good investment of effort for me.  That being said, this did show on some of the evaluations...