An unforgiving spirit is one of the most common of sins. God will not forgive us unless we forgive others (Matthew 18:34-35). It was this sin that the elder son of Luke15:28-30 struggled with in his personal life. Jesus taught Peter that one should forgive not merely seven times, but seventy times seven (Matthew 18:21-22).
Thus, it is clear from this passage that forgiveness can not be reduced to some definite formula. The scriptures teach that for us to obtain forgiveness from God we must be willing to confess our sins (1 John 1:7), motivated by “godly sorrow” (2 Corinthians 7:10).
Thus, forgiveness is extended to the one who acknowledges his/her sin from a place of genuine remorse. At that point forgiveness is to be extended out of our Christian duty.