Watching older movies and television shows today I'm struck by how much longer shots are and how there are so many fewer cuts. I'm sure that, to some extent, pacing in movies was dictated by the practicality of technology. Modern-day non-linear editing and digital production allows for more frequent cuts at a much lower expense. I wonder, however, to what extent there is a feedback loop occurring between audiences and directors that drives shorter shots and more cuts.
WIRED did an article in 2014 that pegs the number at 2.5 seconds. Amusingly, the professor cited in the article has the last name of "Cutting", which makes the article a little jarring to read to me (since I keep seeing the verb "cutting" and thinking about "cutting together" a movie).