How long does it takes for the feeling of vengeance to fade away by time? Psychologically, how long does it take for a person to forget the feeling of vengeance over other person or community? 

For example, imagine this scenario: There was a city which was sacked by army and a soldier enters into a house. That house had a family of 4 members, a married couple with their son and daughter. The soldier killed the couples and the son before the eyes of the daughter. Then he took her as a sex slave in his house for 15 years.

As you know, at the beginning, the girl will have lot of vengeance over the soldier .That is she wants kill the soldier to avenge her family's death. But due to tight security she can't do it.

Now my question is, does the feeling of revenge exists for the whole 15 years OR eventually fade away? If later is true, then how long does it take for the vengeance feeling to fade away?
It depends on the individual (and possibly the trauma). Stockholm Syndrome, where captives develop a psychological alliance with their captors as a survival strategy during captivity can onset very quickly (in the original case 6 days). There are also cases where people are actively trying to escape and do not feel a bond with individuals.