The creator of the manga One Piece, Eiichiro Oda, said during an interview this Thursday that he plans to end the story in four or five years. This information came from Masaki Aiba, who then asked Oda, "Do you already know how many more years the work will have?"
Furthermore, Oda added that "We have to talk about everything whenever my editor changes," and the editor sitting next to him during an interview confirmed that he already knows the ending of One Piece.
Source: ANN