He could have produced a series of short revelatory documents and teachings over an extended length of time — just like the Doctrine and Covenants, in fact. But what happened at first, rather, was the dictation of a lengthy, complex book within the stunningly short period of just two or three months. The Book of Mormon recounts a thousand years of history for a people of whom none of his contemporaries had ever heard. (And throwing in the Jaredites just makes the story longer and more complex.) Joseph Smith's neighbors expected no such thing. They wouldn’t have missed it had it not been provided.
http://www.deseretnews.com/article/8656 ... ormon.html
The methodology used in computing translation time is seriously flawed. The lost 116 pages, according to the text of the Book of Mormon itself, is essentially duplicated in the subsequent version which we now have as First and Second Nephi. That takes up a significant amount of the 275,000 words. The first 116 pages took months to create alone. During the interim period, after the manuscript was lost, of about a year (which Nibley and others don't include in the calculation) Joseph wasn't doing much of anything. He didn't have a job. He could have easily been coming up with text for the book. Joseph Smith's mother stated that Joseph was telling stories about the Indians from the time he was young. The plot and narrative could have been worked on for two or three years or more. Nibley's assertion that it was done in only two or three months is pure speculation at best.
http://www.mormonthink.com/josephweb.htm#speed