On April 5, 1829, Joseph Smith met Oliver Cowdery for the first time in Harmony, Pennsylvania. Just two days later, on April 7, 1829, Joseph Smith began dictating the translation of the Book of Mormon to Oliver Cowdery. By June 30 of the same year, after just 85 days later, Joseph and Oliver had completed the translation of the Book of Mormon, Another Testament of Jesus Christ.
Consider what Elder Russell M. Nelson taught about this:
“Think of the short time Joseph took to translate the Book of Mormon. Working from April to June of 1828, Joseph translated the 116 pages that Martin Harris later lost. Joseph began translating again on Tuesday, April 7, 1829, with Oliver Cowdery as scribe. The manuscript was completed eighty-five days later, on June 30 of that year. Of course, not all of that time was spent working on the translation. The Prophet and his scribes also took time to eat, to sleep, to seek employment, to receive the Aaronic and Melchizedek priesthoods, to make at least one (and possibly two) trips to Colesville thirty miles away, to receive and record thirteen revelations that are now sections of the Doctrine and Covenants, to move from Harmony to Fayette, to acquire the Book of Mormon copyright, and to begin making arrangements for the publication of the Book of Mormon. Conservatively estimated, this left sixty-five or fewer working days on which the prophet and his scribes translated this book, which contains 531 pages in its current edition. (See John W. Welch, Ensign, Jan. 1988, pp. 46–47.) That calculates to an average of eight pages per day. Consider this when you translate a book, or as you schedule your own reading of the Book of Mormon.” (“A Treasured Testament” Ensign, July 1993.)
For some very insightful and informative information on the topic of the translation and the lost manuscript, see the HISTORICAL INTRODUCTION to D&C 10 in the Joseph Smith Paper project.
Sources: lds.org/topics I Have a Question (How long did it take Joseph Smith to translate the Book of Mormon?) http://josephsmith.byu.edu/ History of the Church, Joseph Smith 1:32-35 Opening the Heavens: Accounts of Divine Manifestations, 1820–1844, John W. Welch p. 163 Joseph Smith History A1:13 Doctrine and Covenants 6 Revelation, April 1829–A [D&C 6] Sustaining the Law: Joseph Smith’s Legal Encounters, Gordon A. Madsen, Jeffrey N. Walker, and John W. Welch