Considering trying to write about my life. I feel like I need some sort of unifying theme, a direction. Whenever I've tried to write about my life it becomes rambling and disjointed. Maybe I should take a writers workshop.
The unifying theme is
you.
As with any writing project, I recommend making an outline. It doesn't need to be set in stone at first, but it helps keep the writer organized.
If you're not under a deadline with this, I suggest that you go to a library and have a look through the section on biographies/autobiographies and see what they have in common (structurally).
Some people discuss their lives according to themes, while others start literally on the day they were born.
I own Isaac Asimov's autobiography. It took him three volumes to do it, and they are HUGE. He had a lot to say about himself, his family, his education, and his careers (both of them; before he was a writer and lecturer, he was a chemist and worked at a university). If he'd cared to put in more detail about some things and some of the years, he could easily have gone a fourth volume.
Some who are unfamiliar with Asimov might think, "THREE volumes? What a narcissist!". Well yeah, Asimov did like to talk about himself and his writing and his opinions on a lot of things. He gave a lot of lectures after becoming a writer, and could improvise a talk in very little time. There were very few things in which he was
not interested, as he would do extensive research for his stories. He wrote nonfiction books on a wide variety of topics.
I've read those three volumes twice. That's a lot of reading. But it's the closest most people will ever get to meeting him. The writing style isn't stuffy; he
wanted people to have the impression that he'd just had a very long conversation with them.
That's something you'll have to consider - how formal you want this to be. I've read autobiographies that read like a resume for a job, and that's boring. The ones I've enjoyed are written as though the author is having a conversation with the reader, showing more of his truest self, thoughts, feelings, opinions, and so on.
A writer's workshop might help, or you could check out what writing "how-to" books there are at your local library.
As for the OP question: I'm not sure. There were a lot of times in school when we were told to write about ourselves, but the answer is probably that the longest is in my various diaries. I've gone out of the habit of keeping a diary now, but many years ago it was a daily thing to write something, no matter how mundane it might be.