When does the "leveling effect" happen?

I've heard that it happens if the pKa of the acid is less than water, but then I've also heard it will happen if the acid is too strong (stronger than Hydronium). Is this correct? Are there also corollary rules for what the pKa of the base must be? What are the pKa values to look for to make sure the acid and base will actually react with each other rather than with the solvent?