Question: how long would it take to watch all episodes of one piece?
Answer: The average anime episode is about 22~24 minutes long, including openings and endings, which average to about one-and-a-half to two minutes in length. So 831 episodes x (24 minutes - 4 minutes of OP/ED) = 16,620 minutes, or 277 hours, or about eleven days of nonstop One Piece.