First off the only way to find a Pokemon with its hidden ability(besides hordes) is the friend safari, which you can get Pokemon with hidden abilities. Plus the Pokemon can also be shiny. So... the chance of finding a shiny was halved, so it's 4105(estimate) and a 1/3 or 1/2 chance of finding a Pokemon with its hidden ability(depends on the Pokemon, again, just estimating). So... after a lot of math in my head, I'm guessing the chance is about 3.02% to 6.04%.
Hope I helped!