Certain family members of mine decided to stand against me for custody of my son, apparently because I don't play their game and my ex-husband does. Personally, while I no longer harbor any anger over it, to me it sends a clear message that they no longer have anything to say to me and I can't see any benefit of re-uniting with them. It has been almost four years, and I am very happy with where my life is now without the presence of people who will use or back-stab, and most of the time I don't feel like anything is missing anymore.
So I'll bring in another example. My husband has been estranged from his brother after said brother married their first cousin, and then cheated on her with my husband's first wife. It is now over 25 years after the fact, and a mere matter of months since he finally allowed another family member to give his brother our phone number. There have been no teary resolutions, just, "You're still my baby brother, I'm ready to talk before our lives are over." It will, no doubt, be a long process, and it's very possible that they will never be on easy terms again, but the lines of communication are slowly opening. His brother put the word out that he'd like to talk to him, and my husband basically just let him know that such a talk would be allowed.
I'd say that every case of estrangement will be very different, depending on who was wronged and in what way, or what the conditions of the estrangement were. Individual personalities will also dictate just what shape reunion takes, and how long it takes. In many cases it may never happen, and in some cases it never should happen, but each and every one will be different.