Answer
There is a lot of research behind e-mail response times. That makes it pretty easy to give some parameters.

Most customers wait for some kind of "immediate" response to their e-mail. Otherwise they begin to question if their e-mail got to you and will end up calling you to confirm you are working on their inquiry. So, if you aren't acknowledging e-mails, you may be generating additional calls into your call center. This first acknowledgement response can be automated and should arrive within an hour.

The "answer / solution" to a customer's e-mail request should be within 24 hours (however, if the request is "urgent" then within an hour). This turnaround time is pretty standard for customers these days. I know, as a customer, it is was I expect!