My googlefu fails me so here it is as best as I can remember.
“Every historical trend has three points. The first is when people who pay attention know that something’s going on. The second is when even people who don’t pay attention can’t escape that something’s going on. The third is the arbitrary point where historians say ‘This is where it happened’.”
In “Foundation” there’s this quote “Somewhere in the fifty years just past is where historians of the future will place an arbitrary line and say: “This marks the Fall of the Galactic Empire”
Paraphrased from the opening to “Mother Earth”, published in “The Early Asimov, book 2.” I left out various name-calling and specific examples. Is that more or less what you were thinking of?