This question is usually asked 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.
This is another question you can answer in a short way. You don't want to answer with a single line answer because it doesn't sound believable. So a couple of sentences explaining what you want will convince them that you want to stay for a long time.