I usually respond within a few hours of getting a message, whether it be on the weekend or not. I need every sale I can get, so it's no skin off my nose to take a few minutes to answer a query and more often than not it ends in a sale. If I delayed too long, the buyer might look elsewhere.

That being said, if something comes in after 9pm, I'll respond in the morning when I'm feeling refresh and coherent 🙂