Before we got to Big Mom's arc, (3 years ago) I thought at that point in time the series could conceivably end in about 8 years, by chapter 1200, volume 120, right around the 25th anniversary in 2022.
But then instead of taking 1 year, Big Mom took 2, and didn't even resolve itself, leaving things wide open.
We're now about a year into Wano, and depending on the scale of the battles, I can easily see it lasting another two years. Especially since even if the action resolves relatively quickly in a year or so, the aftermath will still take months.
At this point I can't see it ending before chapter 1400 and the 30th anniversary in 2027, and even that is really tight.
Wano- 2 more years
Stuff we just don't know about. -1 yearReverie, bonus island, vegapunk, shanks, davy back, Rox, Im, revolutionaries, weeviletc. There's bound to be at least one more big surprise arc and stuff that just doesn't fit elsewhere..
Elbaf-1 year
Raftel-1 year (Short overall island adventure, but Including Roger flashback)
War-3 years (1 solid year of which is 10 strawhats getting 3_chapter fight each, plus longer fight for Luffy)
And that's conservative going at top speed. (For simplicity sake I'm estimating 50 chapters per year because that's easier to math, but Oda does about 40 a year now, so really that's more 10 years than 8.)
I totally get that Oda wants to finish and he'll start cutting some corners to get there faster, but then you look at Wano's cast of hundreds and its just… nope. If Oda was really going to make it all shorter he could have cut 10 or 20 chapters from the Big Mom water chase,and introduced less randos in this arc, for instance. But he didn't, because he can't help himself.
I suppose its possible that Elbaf ends up being a really small adventure 10-20 chapter adventure but it feels like it might be where all Big Mom's resolution goes so any chance of it being a 10 or 20 chapter thing is gone. It also feels like the best place roughly to meet with Shanks since after that its Raftel and endgame. And all THAT is assuming the final war doesn't balloon like Naruto and Bleach did and take 5 or 6 years plus a super lengthy epilogue.
To make that calculation even worse, I have to assume that even if he is reaching the organic natural end of the story around year 32 or 33, in the midst of a giant war, his editors might push him to stretch it just a little bit to get to year 35 so it can be a big push that way, and end on some sort of even volume number, the same way Naruto was blatantly stretched to last until year 15. (Though that might be realized by having Oda take more breaks rather than more chapters.)
That said, I do feel like the end is closer to 10 years than 15, which USED to be the estimate I'd give...
One caveat is, Oda might clear out a lot of fodder during this war, which would in turn allow him to make the final war a tiny bit smaller overall, but...