That is up to the individual. Personally, I have done it when I realized whatever it was that was still bothering me, really had no effect on how my life is today. It is wasted time and energy. I have a good life. I have a fairly busy life. I don't have time and energy to waste. Plus by worrying about those feelings, you still give power to someone that doesn't deserve it. I absolutely refuse to do that anymore.
Until the past is separated from the future, it is still present.
Sometimes "letting go" is not sufficient---sometimes it has to be surgically removed and you have to heal considerably to move on.