Although the attention span myth is one of those things that is taken to be almost idiomatic, there’s actually very little empirical evidence for it.

There’s no such thing as an ‘average’ attention span, and even for those individual tasks it would be very much context dependent (where I’m sitting, what I’m doing at the time, my general opinion of the author, the mood I’m in).

It’s a nice story, and it will invariably generate a nice stream of anecdata in a comments section like this, but there’s very little evidence for it at all.