I think this really depends on your role. I don't get enough emails in a day to require 3 hours of replies.
More generally, though, the response can be as simple as "We have received this email; the request will take some time, here's roughly when you can expect an update."
More generally, though, the response can be as simple as "We have received this email; the request will take some time, here's roughly when you can expect an update."