How close can two black holes in a binary system get, before they merge?

Sorry if this question is too elementary, as is my level, but I’m wondering how close two supermassive black holes can get and should the Keplerian rotational frequency at that point equal to the frequency of the emitted gravitational wave? I guess I’m asking how the maximal frequency of classical mechanics of this binary system compares to the maximal frequency of the emitted gravitational wave?