Required as in hard-required?
Zero, I'd say.
I'm not being sarcastic. But you're saying required, which means absolutely required, to me, and the answer is zero. I've run easily 10 hours on zero prep before, certainly zero written-down prep, particularly with PbtA games. I've also run them with many hours of prep.
Do prefer to run with absolutely zero prep? No, haha, I do not.
I think the guidelines you've set out for yourself are generally good though I would agree with
@hawkeyefan that I'd probably go with a larger number of factions, if I intended to run that long, because some of them will become irrelevant or and like, if you only have 3, that's going to get pretty intensely focused on those 3 so they better be extremely interesting, and it limits the opportunities for another faction appearing to cause chaos. I'd also probably detail about 10 NPCs to start with, so long as the system I'm using allows me to rapidly generate NPCs.
That's kind of a big deal - PtbA Or Resistance (Spire/Heart), you can make up NPCs very very fast. But in a lot of RPGs you cannot. Like, Shadowrun it's hours of work to churn out a few non-generic NPCs (well it was in 5th anyway). So if I was playing a game where NPCs were a pain to created, I'd probably create a lot more.
Also I'm naturally prone to making up NPCs, if I wasn't (and I know one of the DMs I play with isn't) I'd probably prep more too (and maybe get a random NPC generator up in there).
The mind-map style approach
@Hawkeye uses is very good for most things. For layered mysteries you can sometimes want to prep a bit more than that, but generally "stuff is going down, get involved/solve it" stuff can be done with a mind-map and quick thinking. One of the latest Spire books, the Magister's Guide (one think I kind of love about Spire is it has actual sourcebooks and they're actually useful but not needed, so many RPGs now either have no sourcebooks or they're absolutely dire and/or needed to make the base RPG work) has a whole section on how approach the game with zero prep, 20 minutes, 1 hour, and 2 hours, and different things to think about.