"How long do you plan on staying with this company?"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.
Short Answers"To tell you the truth, I have been working and gaining experience to get into this company. This is where I want to be and I plan on staying a long time if I'm offered the job.""The only time I get bored is if there is no work to do. As long as there's a lot of work, then I'll be happy and stay a long time. I'm a busy body and I need to do work.""This company has everything I'm looking for. It provides the type of work I love, the employees are all happy, and the environment is great. I plan on staying a long time.""One of my goals is to obtain a position in this company. I know that once I start working here, it'll be for a long time."Long Answer"As far as I can tell, this company has everything I'm looking for. I enjoy this type of work and the benefits at this company are great. I'm looking for a long term position and if there are opportunities for advancement and growth here, then I 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.