In the comments corresponding to the year 345, Mormon notes that by this point in time (at the land of Jashon) he had gone and had taken the plates of Nephi, and did make a record according to the words of Ammaron (Mormon 2:17). Thus, Mormon had begun to record upon the large plates of Nephi which were left to him by Ammaron. Mormon's record on the Large Plates was apparently called as it is in the abridgment, "The Book of Mormon" (Mormon 1:1).
According to Mormon's chronology, the Nephites spent 14 years in the land of Joshua (from 330-344 A.S.). In the year 345 the Nephites were forced to "flee before the Lamanites" (Mormon 2:16-17) and were involved in battles until the year 350 (Mormon 2:16-29). We can reason that because Mormon was the leader of the Nephite armies, the military time period between 345-350 A.S. was probably not devoted to either the study or the writing of many records. However, while previously residing at the land of Joshua, Mormon apparently had about 10 fairly stable years for studying and recording on the Large Plates. [Alan C. Miner, The Chronology and Compilation of the Writings of Mormon and Moroni," p. 2]