It seems to be nearing the end, at least. The big arc that the series has been building up to for the past 10-ish years is wrapping up, and by the time that it's done, the main cast will have almost everything that they need to find the treasure that the series is named after.
Mind you, it probably doesn't help that the ongoing arc has been absurdly long - thanks to having a ton of side characters, subplots, etc., and Oda's output slowing down since he seems to get one break a month in addition to magazine-wide breaks, this one arc has lasted four years. Apparently, it's better to read it all at once rather than reading it weekly, but it's still been dragged out to a ridiculously large extent, even with several fights and other key events seemingly being rushed through.