You may not know the answer to this. If this is your first job, how could you possibly know how long you'll be with the company! When you get hired into a new position, the company invests money into your training, office supplies and your paycheck. They want to know you're going to stick around! Rather than giving them a time frame, you may want to share your career goals. You can say, "I am working towards becoming a sales manager, and I would be thrilled to lead one of your teams here one day." By sharing you have a vested interest in the company, they will feel reassured that you are indeed going to stick around for a while.