What is the longest time that a person has been missing before they were found? I know I have heard of some people who were missing for a few months and then found alive, but does anyone know the longest?

I have heard of people being found as long as 20 years after they had went missing. I think that if I recall correctly that it was a domestic situation where the father kidnapped the child and got away for that long before the mother's private investiagter found them.