I'm looking for a list of all the hidden grottoes in Pokemon Black 2/White 2 and their respective contents. Also, how often do they re-spawn? I've revisited the one I know of and it sometimes is empty.
Each time you walk 256 steps, there's a 5% chance that an empty Hidden Grotto will respawn its contents, so even if you walk a lot, there's a big chance it will stay empty for a while.
Also, here's the Bulbapedia article on Hidden Grottoes, with their locations and possible items and Pokémon.