Alternative and related questions:
If you won the lottery, would you stop working?
The meaning behind the question:
This question isn't just one of silly speculation. What it'll tell the interviewer is what your priorities are outside of your work – and just how much of a priority your work is. That can be very revealing indeed.
Your answer:
I once saw a cartoon which asked the alternative question, "If you won the lottery, would you stop working?" and the reply was, "I stopped working years ago. But I might start gloating if it isn't too hard!"
Many of us think that if we won the lottery we'd end up on a beach somewhere, drinking cocktails and being waited on hand and foot. I have to say, it does sound rather nice, doesn't it? But how long could you really keep that up for before you got well and truly bored?!
There are many ways to answer this question but your focus must be on showing that you'd intend to make constructive use of your time – and don't by any means feel obliged to say that you'd stop working, at least not completely.
Example:
That's a very interesting question. I assume you're meaning a sufficiently large sum of money that I'd be financially independent for life? Initially, I think I'd take a bit of a career break and perhaps go travelling for a while, not least to give myself some thinking time and time to adjust to my new circumstances. After that, I think I would certainly continue working to a degree but I'd probably aim to work on a not-for-profit basis, for example volunteer work for a charity. And I'd certainly aim for a better work-life balance. I have many hobbies such as playing the piano which I would appreciate having more time to be able to indulge. I'm also very keen to learn Spanish. I think I'd certainly find many ways to occupy my time productively.
Word of warning:
Don't venture into the territory of, for example, telling the interviewer just what model of Aston Martin you'd be intending to buy yourself or how you'd quite like a butler to serve your gin and tonic every evening. Restrict your thoughts to ones which show you as someone who needs to continually develop and grow and isn't content to just sit back and rest on their laurels.