The "Long 50s" refers to the period beginning sometime post-WW2, generally said to have ended with the Kennedy assassination in 1963.
It was a time of peace and prosperity, with a strong middle class and industrial base for the United States. And though there was some counterculture, and it was the era of the rise of the teenager and Rock n' Roll, the mainstream was all about keeping your head down and not making a scene (something likely grown of the fact that the parents had lived through Depression and World War, if not fought in it, and now that things were good, they didn't want to rock the boat).
The challenge here is to extend the "Long 50s" period further. It doesn't necessarily mean prevent achievements such as Civil Rights, but overall to keep that 50s feeling era where it was hip to be square for the mainstream, as well as facets of the era if at all possible (such as extreme anti-Communism and faith in the government doing good).