It doesn't really matter what you answer.
Restatement: The specific content is not important, for this and most other answers to interview questions. What we care about is the impression it leaves us about you. "I don't know" says that you aren't goal oriented, "I expect to be a partner making a zillion bucks" says you are insane. Almost everything in between is mostly about presentation.
You need to portray yourself as motivated, intelligent, diligent, interesting, and holding values compatible with the firm. You can accomplish that with a wide variety of answers, depending on how you phrase it and present it. Just be yourself, so long as you are motivated, intelligent, diligent, interesting, and hold values compatible with the firm.
But do know your firm - some firms look for lifers, and will frown on a five-year plan that excludes them. Some hardcore firms will frown on plans that involve family and hobbies. That doesn't mean that you have to say that you expect to be billing 2,500 hours in five years, but don't go out of your way to mention your plan to master the bagpipes.
There are plenty of subject matters in the middle. Nobody expects you to know the future. Law students are notoriously misinformed about their short-term future. We expect you to display traits that we like.
Vague, but perhaps helpful?