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.
It depends on the priority and importance of the email.