Anime News Network reports that Oda says One Piece is 65% done. Oda said One Piece was 60% done 4 years ago. So at this rate (which is actually too optimistic, as the older Oda becomes the more likely he is to slow down), Oda should be done in 28 years. I hope everyone who likes One Piece has 30+ spare years of lifetime left to them, or they won't get to see the ending to the series.
That's assuming Oda actually lives another 28 years. He's already 41. If he lives a healthy life, 69 isn't impossible, but what manga-ka leads a healthy lifestyle? Besides, after seeing what happened to Jun Maeda and Matsu Tomohiro, artists clearly are cursed and are far less likely to live a long healthy life as average folk.
I've said it before and I'll say it again, I don't think there ever will be an ending to One Piece. Like Hunter x Hunter it'll eventually just go on permanent hiatus and then the author will die before ever reaching his conclusion. One Piece will have to be enjoyed as as eternal middle. In return for never getting a resolution, we're treated to one of the longest continuous stories ever written. By that I mean a story where the protagonist never shifts and the plot stays chronologically continuous. In one way, knowing that we'll still get to enjoy One Piece for the next 28 years is the best news imaginable. What's better than opening up the 'newspaper' every Thursday and reading the next chapter of One Piece, neh?
Of course, it's impossible for the current voice actors of the anime to maintain their roles for the next 28 years, so we'll need to recast everyone somewhere down the road. Just something Toei needs to keep in mind as this project continues to roll and roll along. They're also going to need to do a much longer filler arc than the current one to give the manga more of a head start. Unless the anime intends to continue adapting less than one chapter per episode for the next 28 years. Which makes for a much less entertaining episode than early One Piece, lemme tell ya. . .
If One Piece really does run for 28 more years, that's in addition to the current 19 years of serialization. 47 years. That's actually less than many other manga series have run. Golgo 13, for instance, started in 1968 and is still going. That's already 48 years old. So it wouldn't be unprecedented for such a ridiculous event to happen. Japanese clearly like to think in the really, really, really long term. For that matter, Xanth started in 1977 and is still going, so it may well last more than 47 years too. Even western writers are prone to this sort of folly.
I think as a simple courtesy to your fans, you should make sure to finish your series in a timely fashion so that both you and all of your original fans can see the ending of any story you begin. G.R.R. Martin and Robert Jordan could really have benefited from this advice as well. If everyone were immortal I wouldn't mind long stories, but authors have a responsibility to finish what they start, and mortality means there needs to be a little urgency towards fulfilling that responsibility. What Oda's doing is irresponsible and rude to all the fans waiting for his ending. Lacking an ending is as bad a let down as fake-out deaths or revival mechanics. These sorts of things just shouldn't happen. We should have evolved past these obvious faulty story telling devices long ago. Instead it feels like if it isn't one thing it's another. Bleach is canceled too soon, before it can even describe all the bankai in the world, while One Piece runs on too long, boring everyone to death before we even reach Raftel. Can't anyone do anything right? Naruto's manga had a lot of loose ends that needed novels to actually sort out, which are still ongoing, so I can't even give Kishimoto too much credit. He wasn't able to wrap up his story in a thorough or timely fashion either. There were plenty of perfect 10's in gymnastics, but it doesn't feel like there's been even one in storytelling yet. There's always something going wrong. . .