Avoid stating a specific number of years. You might say, “Being career oriented, I’ll be here for as long as I can make a positive contribution to the growth of the company (or serve my customers)”. Or, “I plan on being here to work toward the growth of the business, and my own professional development.” Or, “I plan on being with the firm at least a few years.” You should be careful when making comments about “revolving door environments” or business with high turnover rates to avoid sounding critical of this or other employers. Focus your answer on the benefits of staying with an employer for the long haul, and that you are hoping for a long relationship and career with that firm.