This question tests your flexibility and how far you would be willing to go to land the job. It also tests your honesty, as many might tell the interviewer anything they think they'd want to hear to land the job.
If you weren't aware travel was a prerequisite for the position, ask if it is. Tell the interviewer you're interested in the position and would be open to discussing it further. If your understanding of constant business travel doesn't fit into your current lifestyle, take some time to evaluate how this fits into your long-term career goals. Ask more questions about the position, including if there is room for advancement from within and if a willingness to travel is a prerequisite to growing in the company. Maintain your interest. Keep discussions open and exude flexibility with a focus on a willingness to go to bat for the team. Weigh the pros and cons if you are unsure while exhibiting consideration and flexibility.
