Kent said: "...my feeling is, that if you are not lecturing at least 50% of the time, the students aren't going to learn much."
Are you implying that one cannot learn by reading, experimenting, problem solving? Or is it the "much" that you are emphasizing -- that students who learn by other means do not learn as much content?
I would disagree. I think that students learn, sometimes despite what we do in class, by their own effort. In New York State we have to finish the textbook for the Regents exams. Therefore, "covering the content" is a the predominant concern, unfortunately. If the students are not engaged in constructing their understanding of the content, however, very little of what they learned for the test is retained.
In Schoenfeld's research on problem solving, he noted that mathematicians can reconstruct forgotten knowledge, while students relied on what they could recall. That is, if students could not remember the formula for the sin(2x), they give up on the problem. The experienced person (I hesitate to say mathematician because this would preclude school math teachers), however, reconstructs this from something they do know (perhaps sin(x+y)).
I bring this up because in order to be able to reconstruct something you have forgotten, you much understand it at a level seldom attained by listening to a lecture. I often lecture, but I realize that I am covering the material, which is not guarantee my students are doing anything other than taking notes. In fact, the higher the level of the course, the smaller amount of material actually is digested during the lecture. When I was a grad student, we often joked that we were merely a vehicle for converting the prof's spoken word to written words in our notes. The content was digested later when we tried to read our notes.
A lecture in a darkened room is deadly.\ Eileen Schoaff Buffalo State College