I usually take 9mgs. Any less is not enough, but any more doesn't help anything.
I think melatonin is a great supp for a sleep aid. Works better than any OTC pill I've ever tried. The problem with it is that you can't take it with any real consistency or it will stop working. If I take it for even three or four days in a row, it becomes completely ineffective. But for the occassional use when you can't get to sleep, it works like a champ, with no "hangover" effects either.