Be honest with the interviewer. Maybe you have few personal obligations and welcome overtime. On the other hand, perhaps you have children or a second job. If the latter is true, let the interviewer know that, while you have some obligations outside of work, you can work extra hours, as long as you have advance notice. If that's not the case, and you absolutely cannot work any hours outside of your regular schedule, say so. If you don't, and you refuse to work overtime a few months after being hired, it's likely to create an issue.

"I prefer sticking with my regular schedule, but I understand there are times when everyone needs to pitch in to meet a deadline. As long as I have a few days' notice, I can work extra hours during the busy season."

"I'm happy to work as many hours as needed! At the moment, I don't have many obligations outside of work, so it's not a problem."

"I have several family obligations and a second job, so it would be difficult to work outside of my regular schedule. If asked, I would try to accommodate the request, but it may not always be possible to do so."