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.