The time of preparation is four years. In the example for Ixtlilxochitl, ten years was allowed, so this does not seem foreign to Mesoamerican practice (David A. Palmer. In Search of Cumorah. Horizon Publishers. 1981, p. 209).
Chronology: The last date we had was three hundred and eighty years in Mormon 5:6. Now four years have passed, and we are in the three hundred and eighty fifth year (the three hundred and eighty fourth had passed away), which would be 376 A. D. Mormon would be seventy five years old at the time of this final battle.