It's very low. That's the problem.
The primary difficulty with natural products is that there's no control for quality. Three milligrams of melatonin from one brand can be very different from three milligrams of melatonin from another brand. That's the primary difficulty.
Overall, the studies that have been done tend to be negative or to show very small effects.
Anecdotally, there are people who swear by these products, and it may very well work for people, but we don't have the science to determine the quality of different natural products or to be able to identify who is more or less likely to respond to any of these products.