Interviewer usually asked this question to the people who move between jobs frequently. They do not want to hire someone who will leave after several months. The hiring process costs the company money and time and they want to hire someone who is looking to stay for awhile. I recommend saying you want to stay for a long time. Even plans change, but you want to make sure that you are honest about what can be expected of you. If you plan on it being a long term position say so.

This is the job opportunity I have been looking for. This is the career path I've been waiting for. I ready to serve you as long as the company needs me.