How Fast Should You Reply to Customer Emails?

As email becomes more and more of a customer service tool it is important to treat it just as important or more so than the phone.
Remember to listen to my free tele-conference on using technology for customer service
Ask Dave Taylor writes this answer The solution? I believe that the key is to set realistic customer expectations. If you can’t get to most messages, then say so, with a notice like “Please check through our FAQ and knowledge base: since we’re a small firm, we cannot respond to all email queries we receive.” You’ll note that on this site, Ask Dave Taylor, I do exactly the same with the message on the Ask a Question page for just this reason.
If you can get to customer queries or have staffed the position, then you have latitude and can have a message like “you will receive a response to your query within 24 hours” or similar. But if you do that, make sure you really can answer queries within 24 hours.