How fast should you move when you sense you've made a hiring mistake? In a word, very. So fast, in fact, that if you're moving at the right speed in taking care of a hiring mistake, it will probably feel too fast. That's O.K. In every case, a rapid intervention is better for the organization, your own career, and even the person you're letting go.
Look, hiring great people is brutally hard. New managers are lucky to get it right half the time. And even executives with decades of experience will tell you that they make the right calls 75% of the time at best.