Forgiveness is fundamentally a moral relation between self and other.
Are any wrongdoers unforgivable? People who have committed heinous acts such as torture or child molestation are often cited as examples. The question is not primarily about the psychological ability of the victim to forswear anger, but whether a wrongdoer can rightly be judged not-to-be-forgiven no matter what offender and victim say or do. I do not see that a persuasive argument for that thesis can be made; there is no such thing as the unconditionally unforgivable. For else we would be faced with the bizarre situation of declaring illegitimate the forgiveness reached by victim and perpetrator after each has taken every step one could possibly wish for.
There is no mathematically precise formula for determining when it is appropriate to forgive.
[Charles L. Griswold in Nytimes.]
However, the psychological ability might impede forgiveness anyway, for any amount of time, which is not unlikely when thinking about the heinous act committed by Jared L. Loughner this past weekend.