And would the optimum figure -even for any given person on any given topic- be the same today as it was ten years ago, or in ten years' time, given changes in societal time pressures and attention spans?

Or, for that matter, will it vary depending on the device they use to access the article? I will tend to avoid longer pieces if on my phone, for example, but might happily read them on my desktop. The author can't know what interface will be used.