A few years, really. That’s when I noticed a serious schism on /pol/ that didn’t go away. Generally speaking there was always some level of dissension from the cultural milieu found on the board (this is specific to /pol/, for the most part), but it was pretty small and the overarching theme or opinion was always pretty apparent.
The inflection point was sometime after Covid, specifically with regards to the Ukraine war. And even that divide wasn’t abrupt; it either grew over time or was brought in well after the start of the war. And it didn’t stop there. Every big issue from then onward saw a massive divide without much consensus.
This is why I lend at least some credence to the theory that it wasn’t Soyjack Party that took down 4Chan, but rather a government agency. They tried to control the narrative via shills, and that didn’t work, so they shut it down (for real this time).