So just checked the archive, it's been 1 year since chapter 1129 came out(chapter where Rodo stripped Nami), and since then we've gotten 33 additional chapters. That means we get about 100 chapters every third year just like the Native Genocide Festival. One Piece has at least 300 chapters left, I think we can all agree on that.

So basically we have MINIMUM 9 more years of One Piece at this rate, but I would honestly assume 12 years since I think we still have about 400 chapters more until One Piece is finished, plus Oda might slow down even more at some point.