First, if you’re a confirmed workaholic, this question is a softball lob. Whack it out of the park on the first swing by saying this kind of schedule is just your style. Add that your family understands it. Indeed, they’re happy for you, as they know you get your greatest satisfaction from your work.
If however, you prefer a more balanced lifestyle, answer this question with another:
“What’s the norm for your best people here?”
If the hours still sound unrealistic for you, ask, “Do you have any top people who perform exceptionally for you, but who also have families and like to get home in time to see them at night?” Chances are this company does, and this associates you with this other “top-performers-who-leave-not-later-than-six” group.
Depending on the answer, be honest about how you would fit into the picture. If all those extra hours make you uncomfortable, say so, but phrase your response positively.
Example: “I love my work and do it exceptionally well. I think the results speak for themselves, especially in …(mention your two or three qualifications of greater interest to the employer. Remember, this is what he wants most, not a workaholic with weak credentials). Not only would I bring these qualities, but I’ve built my whole career on working not just hard, but smart. I think you’ll find me one of the most productive people here.
I do have a family who likes to see me after work and on weekends. They add balance and richness to my life, which in turn helps me be happy and productive at work. If I could handle some of the extra work at home in the evenings or on weekends, that would be ideal. You’d be getting a person of exceptional productivity who meets your needs with strong credentials. And I’d be able to handle some of the heavy workload at home where I can be under the same roof as my family. Everybody would win.”

I think that all people should have the ability to choose what they want. If someone want to work on weekends, so why not? Personally I don't want to work on these days because I think that such days should be dedicated for family. Anyway thanks for the great discussion.
Lenny from buy zoloft website