For all the obvious reasons such "super-Assemblies" were few and rare. My own parents attended the Polo Grounds/Tent City NYC Yankee Stadium Intn't Convention and spoke about the long caravan to New York from NM. I have some photos and 8mm film of the assembly.
I remember fondly the Assembly at the Astrodome in Houston Texas where the My Book of Bible Stories was released, but I am not sure how many days that was. (maybe that was only a 4 day one).
Anyway, pre-1975, the JW culture was entirely different, and to paraphrase Whalen, Armageddon was considered just around the corner. So the material presented met an audience that was much more tolerant and perhaps even eager to hear it.
Since the 1980s, the Society has entered a carousel of regurgitation with nothing new in terms of theology being presented to the flock (except for disgarding of a few cherished doctrines and cultural aspects). The 3-day conventions are already bounding on the verge of unbearable repetition from Day One to Day Three today so I can't imagine how hard it would be to fill 8 days worth of material without being accused of falling into a cultic tactic of mindless rote and ritual.
Having said that, from a social perspective, I always wished the assemblies were longer when I was an active JW. This is because, unless I had a hot start (which only happened a few times for me) it usually took me all of Day One to spot and locate the girl or girls I wanted to meet. Day 2 and Most of Day 3 building up the courage to talk to them and by the Lunch of Day 3 if I was lucky, I finally got a name. If I had had 8 days to work with oh boy that would have been great.