Answering your own question is great! There's a whole official blog post dedicated to this: It's OK to Ask and Answer Your Own Questions.
Most of all, it's useful to people in exactly the same situation as you: the ones searching for an answer, and not finding one.
The only real guideline, IMHO, is that the question and answer both need to work in their own right -- just like any post on the site. The question needs to be a real question -- and maybe other people will have better answers to it than yours!
Another thing I've had happen is that the act of phrasing the question can help you realize that your question didn't really make a whole lot of sense, or maybe it has a very obvious answer.
For example, you might start out with, say, "I can't write," and then you go, "gee, I need to be more specific," and then you wind up with "I don't have time to write until 10:00pm and then I'm exhausted." Well, you might feel like you know how to fix your schedule now that you've realized what the problem is. Or like this has turned into a time-management question which isn't really on-topic. In these cases, when the act of clarifying the question kind of invalidates it, I'd give more consideration to just letting the question drop.