Well, we all have experienced a wide variety of SR amounts.
From somewhere around 100 I believe, to over 20,000, or maybe 50,000, depending on whatever Dark Steelix had said it took him.
So it can be any time, but it's most commonly within 20,000 resets that they are found. 20,000 is even a stretch, most people say within 10,000, but I went 12,000 without any shinies...
So it could be ANYTHING.
I'm quickly sprinting through Diamond trying to SR for a shiny Dialga and then Uxie, moar shinies for me!