I think it is good netiquette to wait at least 15 minutes to reply to an email. What do you think? Is that too long? Maybe 5 minutes for professional email within your department and 15 minutes for email that are outside your department or company? 30 minutes for educational email?
If I happen to be online and an email appears that I can very quickly deal with I would reply straight away.
I don't think you should expect to respond to all emails as quickly as you suggest - certainly promptly but not that fast - you would never get anything done!
You need a routine that will allow you to respond appropriately but also allow yourself blocks of uninterrupted time to work.
That is a great point. The reason I wrote to wait a minute to respond was to allow yourself time to respond to all questions raised in an email. I have found that this prevents unnecessary threads for me. It gives me time to get some work done without responding to emails all day.
Email can be a huge time sink. I had a manager once who wrote emails all day. Then stayed late to do the work because that was the only time to work uninterrupted.
It depends on the priority and importance of the email.