Here in suburban NJ, my memory says the cocktail hour might be followed by an optional dinner at someone's home or a restaurant, or maybe theatre tickets, but usually not. This kind of structured and timed event allowed women to dress up in wearable and dressy mid-calf length dresses, and the men to stay in the same dark colored suit they had worn to work.

A 6 TO 8 o'clock event allowed for a break from routine, and an evening early enough to pay the high school age baby sitter and get them home by 9:00 or a bit later.

Mostly, I think cocktail parties stood alone, because everyone with kids had stuff to do in the morning!

I would imagine city events were different.