Sometimes forever. I've had people do some incredibly cruel things to me, and still miss them from time to time, enough that it's like tearing Terre wound open afresh. I crave the good things about them, and the way they made me feel when loving on me. It takes time to remember how malicious they turned out. Or how they betrayed my trust.
One thing that really helps I found is accepting those feelings, that I have them, and don't need to act on them, but I don't have to deny them and act tough either. In time the pain passes, and I'm ok again. I hope this helps