The length of time will vary depending on how experienced a person is with the practice of forgiveness, how deeply the person was hurt, who hurt the person, whether or not the other person is still being hurtful, and whether or not the other has apologized and seems sincere in that. Even if you are faced with the situation in which you have not practiced forgiveness much, you are deeply hurt, by someone who is supposed to love you, with on-going injustice and no apology, it still is possible to forgive. It may take six months; it may take a year. Yet, you likely will sense the progress being made (reduced anger, a confidence in the process, as examples). So, do not think about forgiveness as “all or nothing.” In other words, you will not need a year before you start to feel some psychological relief. The positive changes can be rewarding and increase your motivation to keep on the forgiveness path.
Even though it may seem natural to you, your getting angry with God (over injustices which you experience from people) is not good theology. If God is all holy and sinless, then your forgiving God implies wrongdoing. I prefer keeping a sound theology and understanding that God allows for the free-will actions of people, even if those actions are unjust. People are the ones who behave badly, not God. Rather than forgiving God, I suggest that you try to practice acceptance of what is allowed and then to forgive persons. In this way, you do not diminish the attributes of God.
Yes, I do think that at times pride can lead to such a statement. We have to be careful, however, because some cultures and faiths require an apology prior to forgiving. If pride is blocking the forgiveness process, it might help if the person requiring the apology contemplates this question: “Are you hurting yourself by insisting on the apology? Might you be preventing yourself from reducing resentment and being set free from emotional disruption as you wait for a prior response from the other?”
Not being able to empathize with your sister today does not mean you will never be able to do this. Empathy can open the door to compassion. Sympathy, or feeling sorry for her, also may be such a door to the eventual development of compassion. Yet, as you are seeing, empathy is the deeper, more challenging perspective. Here are some questions that might help you with empathy toward your sister: Was your sister hurt by others some time in the past? How deeply was she hurt? Is she still carrying those wounds? Can you see your sister’s struggles in life? Your answers may induce a greater empathy for her as you see her wounds from her perspective.