How long does it take before you let something go/get over something?

If someone pisses you off, or makes you sad, betrays you, breaks your trust, isnt there for you when you needed them the most, how long does it take you to get over it? “Forgive and Forget” if you will. Or do you never let it go? Why/whats your reasoning for doing you do what you do? Weather its letting it go or not letting it go.