Earlier quoted context omitted.
Compared to classical music certainly, but between the 60s and the 90s the average song length did increase again: https://www.vox.com/2014/8/18/6003271/why-are-songs-3-minute...
Almost certainly in large part due to "Stairway to Heaven" and "American Pie."
While this is admittedly rather handwawy, even outside of this I have the impression that there must have been some lengthening: It feels like basically every Rock & Roll song from the 50s/early 60s mostly is around the two minute mark and very rarely even approaches three minutes. Afterwards however, three or four minutes doesn't seem that unusual to encounter, never mind those really long songs, too.