This question can be a little misleading at times. But I often ask it anyway, to see how people react to it. It can tell you a lot about their perspective on work/life balance and how separate those two things really are. Some people never seem to think their efforts are enough, whereas others seem to think there is a line where “enough” exists and they craft their efforts to reach that line, but avoid crossing it.
On the fringes of this question/answer, there is a personal sense of work ethic and motivation that can clue you in, to what makes a person tick and what they find important. A great follow-up question is “how do you know when you’ve done enough?”