This is a bit of a tricky question. You obviously want to demonstrate you are committed to investment banking but you don’t want to come across as obviously disingenuous by stating that banking is the only job you’ll ever want to do. If you are interviewing for an Analyst position, I don’t think you need to be committed to banking for the long-term (since being an Analyst position is a 2-year position). I would mention that you are really excited about and committed to becoming an Analyst and that you want to learn as much as possible, get as much experience, etc. while you are an Analyst. But I think it’s okay to say that you’ll see what happens after your Analyst position is up (i.e. going to business school, moving on to other jobs like private equity or hedge funds, etc.)
If you are interviewing for an Associate position, then you need to demonstrate a little bit more commitment to banking. I would definitely recommend stating that you see yourself as a banker for the foreseeable future (call it 3-5 years). However, I don’t think that you need to state that you are certain to be a banker for the rest of your life but I wouldn’t say that that is out of the question either.