In "Greasy Lake," when was it that “it was good to be bad, when you cultivated decadence like taste”?

The exact time of the setting is not established, but the general setting is sometime in the late 1960s. One clue is that Bobby's parked car is a 1957 Chevy, which was an especially popular model for teenagers in the 1960s. Also, the story contains a reference to Khe Sahn, an allusion to an important battle in the Vietnam War in 1968. The 1960s was the decade in which teenage rebellion became more prevalent, less shocking, and even culturally "fashionable" among some, like the narrator and his friends.