Meandering down this diversion a bit...

First, yes, I generally agree with you.

What I've noticed listening to a lot of lectures over the past few years are typically breaks at roughly: 90s, 5m, 10m, that 20--30m spot, 45--60m, 90m, and very occasionally a longer session, though I'd argue that 2h is about the maximum which can be sustained in a single session.

The shortest are just teasers, often commercial broadcast. They're not informative but more advisory, and really only advertisements for some additional content, which is the medium's strength.[1]

A 4--10m segment is a brief introduction to a topic, beginning with about the span of a noncommercial broadcast, and including the typical length of a quick YouTube video. You can actually cover quite a bit of the basics of a topic, or mesh together a number of topics into once related thread.

At 20m you're hitting one of the natural spans of human focus and the point at which attention starts to wander without some form of internal break or diversion. Longer presentations are often a fusion of 2--3 15--20 minute segments. I note that many well-structured podcasts aim for about this mark ("History of Philosophy" by Peter Adamson comes to mind.) If you're aiming to fill a 30 minute timeslot, 20--25m allows a bit of additional padding on either side for other filler as well.

Lecture presentations tend to run variously about 20, 30, 45, 50, or a full 60 minutes. Typically there's a Q&A session, and in an academic context, an introduction which I've timed many times at very nearly precisely 6 minutes. The meat of the lecture tends to run about 40-50 minutes, and again, that's a reasonably good intermediate introduction to many topics.[2] And of course, this falls squarely into standard one-hour scheduling blocks utilised by broadcasters and in scheduled event venues.

Most longer lectures I encounter tend to still limit the actual principle presentation to ~50-60m, with a longer Q&A. London School of Economics lecture series comes to mind.

Longer single-session lectures are almost always poorly-tailored and should have been split, edited, or both. I've found a few exceptions. Thorstein Chlupp can in fact talk quite informatively about net-zero residential design and construction in Alaska for upwards of two hours,[3] and Joseph Tainter's "Collapse" talk runs ... about 70 minutes, not terribly long, but over the 1h mark, without feeling thin.[4]

A well-prepared author seems generally to have a 1, 4, 10, 20, and ~50 minute versions of their talk. As I've often stalked speakers over various presentations, I've noticed cut points, transitions, and additional material usage practices.

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Notes:

1. To a first approximation, broadcast media offer infinitely expandable audience but strictly limited depth, whilst print is precisely the opposite. This means that the comparative strengths of each are in providing widespread awareness vs. providing in-depth exploration. Digital somewhat straddles this distinction.

2. A poorly-prepared speaker or interviewer is almost always starting to pad and tap-dance heavily by this point.

3. https://youtube.com/watch?v=AtHkvpRI6fc (2h) and https://youtube.com/watch?v=Xen_VWyDezY (2h36m!)

4. https://youtube.com/watch?v=G0R09YzyuCI