According to an interview of the creator of One Piece, Eiichiro Oda, in "Dai One Piece Shimbun", the manga is about 65% complete. One Piece was launched in 1997 in Shueisha’s Weekly Shonen Jump magazine, while in an earlier interview in 2012, he had stated that it was 60% complete. That means, in the last four years the story progressed only 5%.

Oda added that he cannot do any more long stories besides One Piece. And by the rate that the story’s progressing, it still has years to complete.

The anime series aired in 1999 and is currently in the 751st episode. The manga also inspired many films, with the last (One Piece Film Gold) running on theaters on July 23, video games, novels and more.

Source: animenewsnetwork.com