Can you tell me how long Christ was with the Nephites in Americas?
The specific amount of time that the Savior spent with the Nephites following his resurrection is not given in the scriptures. This much we do know. In the Book of Mormon, 3 Nephi 10:18-19 we are told that soon after the ascension of Christ into heaven he did truly manifest himself unto them– Showing his body unto them, and ministering unto them.
During this visit Jesus taught the Nephites as he taught the Jews, and we have recorded in the Book of Mormon a number of the very same sermons that are recorded in the New Testament. In addition, as we read in 3 Nephi 26:3,
And he did expound all things, even from the beginning until the time that he should come in his glory–yea, even all things which should come upon the face of the earth, even until the elements should melt with fervent heat, and the earth should be wrapt together as a scroll, and the heavens and the earth should pass away; And even unto the great and last day, when all people, and all kindreds, and all nations and tongues shall stand before God, to be judged of their works, whether they be good or whether they be evil.
But more than this, wonderful experiences between this righteous people and the Savior are recorded that appear nowhere else in any of the scriptures.
This first visit of the Savior to the Nephites apparently lasted for three days, but we are told that he returned a number of times to be with and teach the people, as recorded in 3 Nephi 26:13–
Therefore, I would that ye should behold that the Lord truly did teach the people, for the space of three days; and after that he did show himself unto them oft, and did break bread oft, and bless it, and give it unto them.