Just before Nephi notes that "thirty years had passed away from the time we left Jerusalem," he mentions that "we lived after the manner of happiness" (2 Nephi 5:27-28). A few verses later, he notes that "forty years had passed away, and we had already had wars and contentions with our brethren" (2 Nephi 5:34). If Lehi's group arrived in the promised land after approximately 14 years of travel (see Appendix A), and if, according to the same chronology, Nephi fled from the land of first inheritance after seven years, then the Nephites in the land of Nephi might have had a period of peace of around 9 to 19 years before the Lamanites began to bother them. [Alan C. Miner, Personal Notes]
“We Lived After the Manner of Happiness”
According to Brant Gardner, Nephi does not say that they were happy. He says that they "lived after the manner of happiness" (2 Nephi 5:27). In Nephi's terminology, this would mean following the path of the Gospel as Nephi understood it. Nephi's use of the word is probably a result of Lehi's understanding and use of that concept:
And because of the intercession for all, all men come unto God; wherefore, they stand in the presence of him, to be judged of him according to the truth and holiness which is in him. Wherefore, the ends of the law which the Holy One hath given, unto the inflicting of the punishment which is affixed, which punishment that is affixed is in opposition to that of the happiness which is affixed, to answer the ends of the atonement. (2 Nephi 2:10)
Lehi considers happiness one of the eternal results that is affixed to eternal law. The obedience to Law produces happiness as a result. [Brant Gardner, "Brant Gardner's Page," [http://www.highfiber.com] /~nahualli/LDStopics/2 Nephi/2Nephi5.htm, p. 30]
Note* Thus "the manner of happiness" relates to covenant obedience, a major them of the Book of Mormon. [Alan C. Miner, Personal Notes]