As of now according to the author/artist (eiichiro oda) One Piece is just a little bit over halfway through it's story; and if you calculate potential breaks or hiatuses into the equation…good god, when One Piece ends i'm almost going to be in my thirties! O____0

Sometimes, I joke with my friend that 10 or 15 years from now, it's finally going to show Luffy and crew finding One Piece, and then afterwards showing "END OF PROLOGUE", and that everything up to that was just set-up for the actual story which hadn't even started yet.