Write emails that be answered in one word ("yes" or "no") and you'll receive responses much faster.

See also: Why some emails go unanswered - http://theoatmeal.com/comics/unanswered_email

To the people saying "4-5 hours" for a response: I would really disappoint you. I check my mail pretty regularly[0] but make a point of not responding too quickly to many things because I do not want to say something in haste.

Sure, if there is a time-factor involved I will try to answer faster, but the limiting factor is rarely the frequency of my checking mail but my desire to be more thoughtful in my reply.

When sending mail I assume the same is true of others. They may have gotten my message right away but might want to put some thought into their reply. For most things I expect an answer in about 24 hours.

[0]: When I am heads-down working I put off checking E-mail because it is distracting. I'll skip it for several hours. Clients and friends will call me if something is urgent.

On the same page in terms of checking emails frequently, but not necessarily responding immediately so that I can formulate a more complete response / address it fully when I have the time. That being said, what are your thoughts when someone takes a little too long to respond? Do you actively notice when people take longer than anticipated?

Oh yes. Some things I know take time, but if I don't hear back in a day or so I start getting antsy :). But I dislike being a pest, so I set a reminder for myself to follow-up after what feels like reasonable amount of time, and try to phrase my follow-up in way that doesn't come off as "Hurry up!"

I typically expect an answer within 2-3 days. Varies depending on whether we're actively working together on something, whether there are upcoming deadlines, whether we have a formal work relationship or it's an outside inquiry, whether it's a quick 30-second answer or should have some thought put into it, etc.

someone had posted an article about this awhile back, but (let's just say it could be implemented easily, I know it's unrealistic), having an email protocol system with different levels of priority, so that emails that are time-sensitive have higher priorities than marketing emails? I'm not talking about filtering emails into labels like google does with it's inbox/social/promotions, but actual priority levels that you'd add similar to an email's subject line. And if filtering is a more realistic option, what about establishing some sort of standard for adding priority to the end of an email (ex. ::::Important:12hours::::) that a filter would then pick up. And abuse of the system would result in your future email's priority not being respected as seriously by whatever sorting algorithm.

as fast as possible

sometimes that's seconds, other times it's weeks. (oops)

i figure 4-5 hours is the longest anybody is away from email. maybe slightly longer for super busy or offline people

About 4 hours if I know the reader is in the same timezone and its during business hours. If I don't know the location then its 24 hours.

Within 24 hours 99% of the time. Longer for unusual circumstances.

Although always preferrably asap