Not OP but I read the Slate Star Codex article, and buried about halfway down was:

"These don’t “cure” the condition permanently; you have to keep doing them every day, or your circadian rhythm will snap back to its natural pattern."

So it looks like you have to take the melatonin on a continuous basis.

I only started a few weeks ago so I'll let you know, but I can say so far I haven't been militant about using melatonin every day and already I'm still able to go to (and fall) asleep at the earlier time.