Unfortunately, it's impossible for anyone to determine when you'll get a shiny without rng abuse or exploits.
The odds of finding a shiny range from anywhere from 1/100 to 1/8192. I'd recommend reading this for more information.
To explain probability, it's best to use dice/a die as an example. When you roll a die, you have a 1/6 chance to land on the number you want, in this case, let's say you want a 3. You can roll it 6 times, since there's 6 sides, but it's still possible for you to not roll a 3 in those 6 rolls. You could roll it 20 times and still not roll a 3. Likewise, you could roll it and land a 3 on your first try.
In Brilliant Diamond, the shiny odds are 1/4096.
Now, imagine a die/dice with 4096 sides. Finding a shiny is like wanting one particular number between 1 and 4096. You could roll the die forever and never find the number you want (the shiny). You could also also roll it and get the number you want on the first try.
It's all up to luck.
I hope this helps!