I have always just jumped right into my writing whenever I have an idea that I fall in love with. But I get about a hundred pages in and I'm out of ideas. I know that's just that middle slump but that's when I realize how two dimensional my characters are and how basic my plot is. At that point I usually just can't seem to write anymore.

This time I'm doing it different though. I've been figuring out the characters and working on worldbuilding a little bit at a time for a couple of weeks now. Sometimes I write a little short story that takes place in the characters past. I haven't gotten the main characters figured out completely and there's still a lot I need to do but I was wondering how to tell when it's time to start the story? When do I need to tell myself to stop planning and start writing? I know everybody is different so here's my real question:

How long did you plan out your story before you started writing? And how did you know you were ready?