Dungeons and Dragons

Uncountable.

A single intense session can they're take an entire weekend of planning as your primary task, depending on how you run your table. It can take longer.

I remember the big fight from my longest campaign ever.

Arranging tokens, having charts set up for pre-rolled checks for the most common bad guy actions, drawing up maps, planning out the "big" minions' actions, the actual bbeg's actions he intended, the kinda of minions and their sections of books marked and ready, organizing miniatures for the sections that would make use of them, checking character sheets to make sure I could keep the important parts in my head, and more.

I think I worked for two weeks getting it all set up, an hour or two a day mostly, with the two days before set aside for getting everything finalised and physically set up, and those two days were pretty much an all day thing.

And that's with me already having had a good chunk of that info in my head. Back then, I could quote off most of the recurring monsters' stats, and the main bad guys too.