One reason interviewers ask this question is to see if you're settling for this position, using it merely as a stopover until something better comes along. Or they could be trying to gauge your level of ambition.
If you're too specific, i.e., naming the promotions you someday hope to win, you'll sound presumptuous. If you're too vague, you'll seem rudderless.
BEST ANSWER: Reassure your interviewer that you're looking to make a long-term commitment.that this position entails exactly what you're looking to do and what you do extremely well. As for your future, you believe that if you perform each job at hand with excellence, future opportunities will take care of themselves.
Example: "I am definitely interested in making a long-term commitment to my next position. Judging by what you've told me about this position, it's exactly what I'm looking for and what I am very well qualified to do. In terms of my future career path, I'm confident that if I do my work with excellence, opportunities will inevitable open up for me. It's always been that way in my career, and I'm confident I'll have similar opportunities here."