Did Oda say when he will finish his work One Piece? Why has it been going on for so long?
Most of the time the ending date of a serialized manga can't really be known until the last few chapters, alternatively the start of the last Arc of the series.
As long as Oda enjoys drawing it and it keeps being a cash cow for both him and Shueisha it'll likely keep going, but last year Oda said that as of now the story is about 65% done. Doing the math it should be going for about 5 more years. But of course this is just a very rough estimate, it will depend on the length of the future arcs, while there's also the possibility that Oda will decide (or will be forced) to add filler arcs.
As for why it's been going for so long, there are two reasons:
- Oda enjoys doing it (and didn't die yet)
- It's a cash cow for Shueisha (so it hasn't been cancelled)
On a side note, One Piece isn't even that long, there are ongoing series with over a hundred volumes out there. The longest running Shounen Jump series, Kochikame finished last year, it was running for a solid 40 years and has 1960 chapters in 200 volumes.