The Great Rubber-Band

I was thinking about how pop-music in the late 20th-Century was becoming far more rigidly allocated, although longer...in time.

Back in the 50s, pop-songs were each about two minutes in length...
By the 80s, songs were about 3:30 on average.
At the edge of the century, songs were edging four minutes in length...

Economically speaking the trend is stretching over the years like a giant rubber-band...whatever that means ;)