Right, if you mean to watch in one sitting, that wasn't something you could do in the '60s. But it's something that seems like it's been common since the '90s, since the advent of the "DVD Box Set". People would get box sets and watch these long-narrative-arc series in marathon weekends, or socially in parties. There was a period in the 1990s when seemingly everyone I knew was organizing Twin Peaks parties, and it seems like it fits the description of long-running experimental novel. People did that with the X-Files too, though admittedly it was a more coherent experience if you cut the "monster-of-the-week" episodes from the sequence (but hey, leave them in, and make it a gigantic, sprawling novel with disconnected subplots, of the Alexandre Dumas variety). Or Dawson's Creek, for that matter, or Buffy as you mentioned. In my circle of friends it seemed everyone was gorging themselves on Babylon 5, Star Trek, Black Adder, and Monty Python box sets as well; I don't know if I'd describe those as narratively complex, but there are lighter novels, too.

But in any case, I could believe the numbers are different. Perhaps box sets were not as major a part of 1990s/2000s TV-watching as I had thought?