As far as I know she can wait as long as she wants. It would be safe to say she has already explored I think.
You realize that the term "exploratory committee" isn't a term of art, but rather a legal entity that allows candidates to raise funds and spend money for certain things without announcing their actual candidacy. right?
Why is this important? Because of the wildly restrictive campaign finance laws, it's difficult to raise money and spend money without either forming an exploratory committee or a campaign committee. In short, they aren't set up to explore squat. They're set up to skirt campaign finance laws, legally.