There are 75 volumes of One Piece + around 2 volume's worth of non-Volume chapters. Recently I found myself to be able to read around 1 volume per hour. But that is taking in mind it was around my 3-4th time reading the manga, and I had already watched the Anime, so I was already familiered with the story.
I think you should take your time, and read every chapter in around 15-20 minutes, but you'll find yourself flying through the chapters in no time, so 10 minutes would do quite easily. There are 765 chapters, so it's around 128 hours. If you dedicate 2-3 hours of your life to read an amazing story, then you should be caught up with One Piece in around 2 months.