What is the rule for how fast you should be responding to emails?
The short answer: As soon as you can.
The long answer: It is obvious that some emails will be more important to you than others. It also is clear that we only have so much time in a day and there will be days where you will be unable to reply to any emails at all. So, you do the best you can to reply to everyone as soon as you can.
Onliners look at email as an instantaneous medium. They know that their email is in your inbox waiting to be downloaded or ready for you to read usually within minutes to hours of clicking Send.
If you are so busy that you cannot respond at all (you’re there but not “away” to require an away message be in place), you are in fact deciding to ignore the Sender — even if for the time being — and that’s exactly what they will assume. You’ve made a decision that their email is not important to you or you would have responded.
There really is no gray area here. Perception is alive and well in regard to how quickly you respond to those who take the time to email you. Of course, this does not include spam or irresponsible forwarders.
That is why it is so important to have a informative away message in place if you are not available at all. Senders will then understand if they don’t receive a prompt reply from you. And don’t use these messages to let everyone know how busy you are and that maybe, you might, if you have time, get back to them when it suits you — at your convenience. That’s just narcissistic and rude.
If you are simply swamped or have other responsibilities that keep you from responding in detail, a short message stating your situation and that you will respond in detail as soon as you can is highly recommended.
Remember, email isn’t just about you, how busy you are or what you feel is important to you at that point in time. There is another human being involved (the Sender) on the other side of your screen who emailed you for a reason and is expecting your prompt response.
Showing the courtesy to keep those you communicate with informed as to your status can ensure you are a person that is looked at as a pleasure to communicate with — and avoid misunderstandings and hurt feelings in the process.