Regarding One Piece, or any long-running anime, I've never been put off by length. For instance, I've managed to reach episode 60 of DB Kai in a matter of days (specifically, three). Of course, I acknowledge that just because I can devote an entire day to watching an anime doesn't mean that everybody has the same stamina. That, and I tend to watch anime while doing something else, which is a bit more difficult for those who rely on subtitles. So, in essence, if we consider how quickly I've watched DB Kai, I can imagine getting through the entirety of One Piece in a few months, and this is assuming I only watch it during the weekend.
Additionally, rather than putting me off, I'd rejoice in the fact that there's over 400 episodes of an anime that I'm enjoying. I've never understood the mindset that because there's a lot of episodes to get through, you'd rather not watch it... Just watch at your own pace and enjoy it. There's no time limit or requirement to finish it quickly.