To just cut and paste what I've said elsewhere.
I understand it was news when Oda gave a specific number but it's just… its not going to be that soon unless he really rushes the home stretch.
I thought 30 years (2027) ~1200 chapters was about right a few years ago, but the last two arcs length makes that seem way too short.
Had Big Mom been a silly little 6 month detour arc, we'd be on track to maybe finish in five years now. But it went for two years AND didn't finish any of it's plot threads, and now it looks like Wano (already a year in) is going to be at least two years itself, if not three.
Just for math's sake, 1 year=40 chapters
Wano-1.5-2 years
Elbaf-1 year
Raftel-1 year
Final war-3 years
That's almost 300 chapters, before you throw in extra subplots (Shanks, Vegapunk, revos, flashbacks, island we've been given no advance warning of, etc) surprises, and that 120 ch. is VERY conservative for the BIG FINALE.
Obviously only Oda knows his plans. Maybe Elbaf and Raftel will be short 10 chapter adventures before launching into the final war, and that will stay tightly focused on the Strawhats... but none of those things sound like Oda's style.
Even something like the Strawhats vs. BB pirates... if all 10 (11?) crew members get extended 1 on 1 fights as Shonen Law dictates, even if they're just 2-3 chapters apiece, that's a solid year by itself. And the main Luffy/BB even being longer (ala Water 7) and well...
–- Update From New Post Merge ---
@FelRes:
and it always ends with a range of like a decade.
I used to calculate it at 15! Once Wano is done I'll feel safer calling a shorter number than a decade, but… seven years is the bare minimum I can see at this point, and that's if Oda really rushes through some of it. No way its actually five unless something really crazy happens.
It never shrinks because Oda takes what we already knew and estimated around, and then makes that arc longer than any arc before it. Dresserossa taking two years was really unexpected, we kind of thought that would be a short stop and Doflamingo would carry over a couple small arcs instead. But...