The interviewer wants to hear that you have realistic career progression expectations within Costco.
Tell the interviewer that you are excited about joining the team at Costco, and you anticipate that you will likely be in the same job in two years that you are hired for. If a promotion or another opportunity is available before then, that is great! But, you recognize that it takes time for developments and internal job changes to occur, so in two years, there is a good chance you will still be in the same job title, and that is okay with you! Be sure to mention that your goal is to learn as much as you can, so when the opportunity arises, you can be promoted.
If you are a student, tell the interviewer that you plan to stay at Costco through graduation in the role that you are hired into, and then share what career or further education you hope to be pursuing following graduation. If Costco offers jobs in your field, such as management, be sure to mention that you would be open to considering a career in your area at Costco if one is available upon graduation.
"In two years I would like to be on track for a promotion to assistant manager. I realize that a move like that takes time and dedication; both of which I am more than willing to contribute."