Studies suggest that learners attention, after a 2-3 minute settling down period at the very start, can be held for about 15-20 minutes Johnstone and Percival studied students with 12 lecturers in over 90 lectures. They spent 2-3 mins settling down, 10-18 minutes first lapses in attention after 10-18 minutes, then progressively shorter attention periods, dropping to 3-4 minutes towards end.
A later study, by Burns, matched student summaries with lecture content and found exactly the same thing , "As the lecture proceeded attention spans became shorter and often fell to three or four minutes towards the end of a standard lecture." Highest recall was at the start with a fall off after 15-20 minutes.
You would think that the solution to this problem would be no lectures, at least shorter lectures or even the recording of lectures for future attention at your own pace. But no, the one long lecture remains the mainstay of almost every educational institution and conference. This is akin to transport by horse, herbal medicine or astronomy by binoculars. They all work to a degree, but are hopelessly limited.