tbh I don't think there really is confirmed odds of a shiny popping up,it's just completely random and luck based. Like someone could get shinies back to back,while someone else could get a shiny after 5 months,or even a year. my luck likes to bounce around and be amazing,to be completely dry lol. currently have 14 shinies to my name! though I will never forgive my luck for giving me a duplicate shiny Infernal Twin and a shiny Ms.Trial