I use melatonin supplements on a fairly regular basis over the past year to regulate my sleep cycle, as recommended by my doctor to counteract Ritalin-induced insomnia. Studies have shown no reduction in efficacy after three months of use, according to Wikipedia[1]. If no change could be detected by that point, then there's a good chance that it's fine for extended use. From my own experience, this has held true.

[1] http://en.wikipedia.org/wiki/Melatonin#Stimulants