There is no single rule that works for everyone, but most clients feel more comfortable when they hear back within a few hours during working hours. It does not always have to be a full answer, even a short message saying you received their note and will respond properly soon goes a long way. The problem is when clients send a message and hear nothing for a day or two, because that silence starts to feel like a warning sign to them. What response time do you aim for with your clients, and do you think it has made a difference to how they see you?