It strikes me that the societal shifts in the 3 years between 1963 and 1966 (in the U.S.) might be the largest cultural change in a small amount of time since this country was founded. Any other contenders?
Perhaps, 1919-1922. The end of WWI, Prohibition, and women’s suffrage sparked radical changes in music, fashion, literature, cinema, and sexual mores.
There’s a lot more to culture than politics, but it seems like between 2015 (late Obama) & 2018 (mid-Trump) we entered a whole new society.
A similar argument could be made for the time from immediately pre-Crash of 1929 to 3 years later in the midst of the Great Depression. Culture is also far more than economics, but a lot of people were leading profoundly unexpected lives by mid 1932. And considering new ideas about how to operate the culture in response.