Answer 1

First of all very small. Almost none because the items regen differently..
Also, different grottoes have different items sometimes available. Also, out of the 2% chance of regeneration, 40% are items, 40% are other items, and 20% are Pokemon. So the chances are small as the items will regenerate multiple times before all of the items are found. And the 2% chance is every 256 steps. VERY SMALL AMOUNT OF CHANCE OF GETTING ALL ITEMS IN A TIMELY MANNER.

Source: http://www.serebii.net/black2white2/hiddengrotto.shtml

Comments

Can you give me an exact percentage

Answer 2

The contents of the Hidden Grottoes are random, and if they are empty,foreach 256 steps, they have a 5% chance to regenerate, usually with different content than what was inside on the previous visit. Hidden Grottoes will not regenerate unless the items are obtained or the Pokémon are captured or defeated.

Source: Bulbapedia