Repentance does not require a time period. Look at Alma the Younger, the sons of Mosiah, and the Apostle Paul. Now these were encounters with God, but so were the conversions of many of the Lamanites. (Alma 18: 40-42; 22: 18, among others.)
The Lord tells you to repent. If you do, He remembers your sins no longer. Confess and forsake them, and you will be forgiven. (D&C 58: 42-43.) Or, in other words, change. Turn away from your sins and face God instead.
All those labors performed by Alma the Younger, the sons of Mosiah, and the Apostle Paul, after repentance, were not to obtain forgiveness. They were the “fruit” of repentance, or the result of the new direction that they were heading. (See Matt. 3: 8; Luke 3: 8; Alma 5: 62; 13: 13; Moroni 8: 24-26.)