In this article Professor Dylan Wiliam, deputy director of the University of London’s Institute of Education, is quoted as saying: “The optimum length of lesson depends on the subject. For some subjects, a short ‘input’ type lesson of 35 minutes might be fine, but it would clearly be inappropriate for art, PE, science practicals and so on. (…) The most important point is not the length of the lesson, but the appropriateness of the activities and the recognition of the need to change focus every so often.”
Which you note yourself in your question.
This research paper (http://txcc.sedl.org/resources/briefs/number6/) reviewed studies on this topic, and they conclude: “The impact of class time lengths on student achievement appears to be a complex issue with no definitive answers. A major theme across many of the studies reviewed is that the amount of instructional time is not so important as how that time is spent. “
A colleague of mine who consistently wins teaching awards claims that he has a short attention span so as a result, his lectures tend to switch things up every 20 minutes or so. Unlike normal university lectures, his are pretty interactive with live examples and interactive experiments the class participate in to demonstrate key concepts in psychology. And the students love it. Equally important, their achievement levels have gone up since he started teaching this course so they’re clearly getting something from it too. So all of this is by way of agreeing with Iroise — I don’t know how much the time of the lesson matters as long as there is enough activity to keep them engaged.
But this is really about pedagogy — I don’t know of any relevant neuroscience to help guide this. I wonder whether teachers would like to chime in with their experience here?