Oda clearly stated One Piece was halfway completed when he reached the timeskip, which would be chapter 597. Just have to double that number and you'll get 1194, which probably means One Piece will end around that amount of chapters.
For those who wondered, this means it will take about 11 years for it to be completed.
I think Oda once said that he has written the whole story, everything with a lot of details, and put it in his testament. So it will probably be given to someone he trusts, in case he dies before the series ends.