A review of Beyond the Mountains of Madness (2021) reprint (re-laid out to current Chaosium book sttandards) for Call of Cthulhu 5E. What caught my attention was the blurb "The review is largely glowing, but warns of the onerous prep for running this adventure." and indeed the review spends a couple of paragraphs detailing the prep required: there's Lovecraft's novel-length story to read, the book is about 300 pages of adventure divided into 19 chapters and there's 140 pages of reference material. And if you bough the companion book (published as a Chaosium monograph) that's another about 150 pages of material.

Still, I did run this campaign as part of GURPS CoC and finished it up as part of GORF. 20 sessions with five or six coming from other supplements because a lot of the BtMoM chapters are quite short. The only prep I did was reading all the material once then re-reading the chapters as they came up for the gaming session. Mind you I do not run very detailed inventory and NPC relationship webs.

So it is a few weeks to read everything but can be done while running another campaign.