When people say "it's a long story" it either means:
1: they don't want to tell you (it's usually this one)
Or:
2: it's actually a complicated story (but they usually explain it in a short way)
The way to make them tell you the story when they say "it's a long story" is:
"We have time."
I read it in a book. I use it a lot. ;)
