Yeah, I agree with Firerain, stick with the chapter method, it's what I always plan to do.
My quick thoughts, 10-20 months, 100-150 hours? Either you have no life at all, or your game is going to lose quality on a major element, like storyline (my main issue with the one game of yours I played, it had lots of content, but it was all low quality to me).
But yeah, release episodes, if you want to see a game that's done it right, check out Master of the Wind or Quintessence. I'd suggest all chapters have at least 2+ hours of gameplay.