The interviewer wants to know how far ahead you've planned. Your plan doesn't need to be rigid or inflexible. In fact, the more adaptable and flexible you are, the more emotionally intelligent you'll seem. Life happens and plans often change. Showing you are aware of this can go a long way here. Having a focus and exploring the possibilities is the aim here. Whether you intend to join an existing practice or start your own, be honest about your plans and desire for your career path, and convey that you've put thought into it, regardless of your decision.