As of XY, 1 in 4,096 is the chance of finding a shiny through Soft-Resetting without Shiny Charm.
Nothing will change that (unless you hack)
You can SR 200 times on Tuesday and get no shiny, but you could SR on a Wednesday and get a shiny. Its completely upto how the RNG dice rolls.
If you SR 50 times for 4 days it does not increase or decrease your chance than you find a shiny compared to 200 in a day.