Usually it's whenever I have time enough to sit down and write them out. You have to have something to say before you say it. This may sound obvious, but rarely is.
Writing is the same way.
Before your pen ever strokes the first word, you should know what you've got to say. If not, the only thing you'll be doing is stringing random moments together, and praying it turns out all right. This may produce semi-decent art, but it seldom produces solid writing. Solid writing is real, and the readers can feel it. It's real to them because it was to you when you wrote it.