If you never burn out, then you are either superman or you don't work that hard. It is ok to admit to burning out at times, and you should provide an answer that shows how you deal with the times when you do get burned out.
As you can tell, this is an easy answer. Many times a person who constantly works without a break can eventually become a bad employee by burning out. Being able to communicate exactly what you do during burnouts or how to prevent them indicates that you have balance. So simply listing a couple of hobbies and adding, 'keeps me energized to work hard in the office' is a good enough answer that the interviewer will accept.