>Typically, when you go to an area, once you've mined it for its interactable Pokémon and items, it is useless, the Hidden Grotto is different. The contents of the Hidden Grottos change and get regenerated repeatedly.Whenever you take 256 steps in the game, there is a 5% chance that each of the Hidden Grottos will be regenerated. When it's regenerated, there's a 40% chance of an item, a 40% chance of a hidden item and a 20% chance of a Pokémon. The chance that it regenerates can be increased using a Pass Power.
So the Grottos, rather than regenerating overtime, regenerate over footsteps. And it's only a 5% chance for every 256 steps! Unless you can access a Pass Power, this isn't gonna go by very quickly.
So in the meantime, do a lot of biking around. A good activity would be revisiting places and making sure you didn't miss any items or breeding a shiny Pokemon with the Masuda Method. (If you have a foreign Pokemon from before the DS Wifi was shut down.) Also, make sure you're checking the Grotto in Pinwheel Forest, as that's the only place where you can find Breloom.
Hope I helped!
Serebii: Hidden Grottos