I’ve found that users really appreciate hearing back from me as soon as possible, even if it’s just to say that resolving their problem will take longer than expected, or that I’m working on it. In the world of web apps, where you can send an email and get a slow response, or even none at all, users find it really refreshing to get a quick response just so they know someone is on the other side, even if the issue isn’t fixed right away.