Since a few days I see people are very happy with +15 Wave. Something has changed that I don't know, because the price of such weapons exceeds the price of even one TS9D?
Random times, also it doesnt spawn a single +15 wave but multiple ones. I myself have destroyed 3 hainan statues so far and I got over 20x +15 waves out of it.
It spawns 2 times a day in a random location and within a random timeslot. Drops can vary, but you will most probably get 5 boxes with wave weapons+15, few scrolls to enter Hainan, and other items from Hainan mobs.