Most companies/agencies want people who are ambitious and are looking for ways to grow. You don't have to answer this question with a specific title/role. It is perfectly acceptable to share what kind of work you want to be doing versus a specific role or title. Depending the company/agency that may be a preferable answer because it suggests that you are focused on the work versus a title/role.

The goal of your answer is to show that you view your career as a journey. You want to communicate your focus on growth and evolution as a designer. This is an area where doing research on the company/agency's work can really pay off. You might be able to tie your career ambitions to the work they are creating.

My long term goal is to be an art director. Over the next two years, I plan to immerse myself in work that helps me evolve as a designer. I'm looking for a position that will give me opportunities to grow. I want to be challenged and have a chance to create amazing design in the process. Beyond that, I want to work with team members who will inspire me. I'm not sure if I will be at the level of an art director in two years time, but I will do everything possible to be on that path.