Historical: This verse makes it appear that Alma and his people were in Helam for some time. The were there long enough to build a city, and long enough to both "multiply and prosper exceedingly."There is no way to know how long a time this was, but it strongly suggests that the time between the death of Abinadi and the death of Noah may have been at least a year, and perhaps more. Additionally, there could have been a year or two pass from the time of the Lamanite victory over the Limhites to their decision to flee.
Since all of these events occur in Helam prior to the flight of Limhi's people, sufficient time has to have passed for the building, multiplying, and prospering to have taken place. One would expect that this is a minimum of two years in Helam, though it could easily have been more. Unfortunately, the text does not give us enough to go on.
Textual: The printer's manuscript has Helaman here which was corrected prior to the first printed edition to Helam. See the analysis which follows Mosiah 18:12 for details.