After one message. Reply to the next three early and your response time will be down to a more normal amount.
Anyway, to answer your initial question, you can't set your hours or prevent people from messaging you at certain times. You just have to be available fairly regularly (in a healthy way) and allow your stats to even themselves out over time. You'll get some messages while sleeping. You'll get some while you're awake. Overall, you'll probably end up with a response time of 1-4 hours, depending on where most of your customers are from.
You can create a new account if you want, but it's extra work, and a similar thing could happen again.