The narrative arrived at the point where thirty years had passed since they left Jerusalem. It is tempting to suggest that the command to create the second set of plates came sometime soon after that thirty year anniversary, because Nephi speaks of creating plates right after.
Nephi says that he had made plates and kept a record, and then that Jehovah commanded him to make other plates. Thus, we do not know when the other plates were begun, just as we do not know when the people of Nephi began. What does appear to have happened is that after they had been gone for thirty years, probably twenty-one to twenty-two of them in the New World, Nephi begins the project of writing on the plates the text that we are reading.
Nephi ends the chapter by indicating that forty years had passed since they left Jerusalem. He does not provide the details of what happened in the intervening ten years, but it appears that at this point Nephi’s writing catches up to real time. From this point on, Nephi isn’t writing according to a story that he needs to tell, but according to stories or instructions he wants to tell for the future.