You should avoid discussing your salary at interview - so conventional interview wisdom goes! In this context, however, the question may be about a rather different issue. Many marketing jobs have the potential for staff to earn bonuses or indeed be on a contract where their salary is only a proportion of their potential earnings.

This question also suggests the future. What are your earnings likely to be in the next five, ten, even fifteen years?

You should relate your earning potential to your ability to do the job. By arguing that you hope to take every opportunity that comes your way, work hard, put in the hours required and generally get stuck in you will show that you are keen to earn as much as possible.