Just like everybody is different, every treatment is different. Here are some links to trials using a variety of treatment block durations.
Melatonin: 2 weeks treatment blocks (sleep in Parkinsons); 4 weeks (sleep in alcohol dependence); duration of hospital stay (delirium in elderly patients); 30 days (inflammation in athletes); 21 days (sleep). More melatonin studies.
Caffeine: 2 and 4 week treatment blocks (insulin resistance);