What are the rules of good style in business email communication, regarding timeline of message exchanging? I mean, how fast the incoming message should be answered? What are the most possible delay in reply without the danger to look impolite, unorganized or even rude?
I believe that different rules should be assumed for different types of email addresses: the rule for firstname.lastname@example.org will probably have more strict rules than email@example.com and maybe more strict rules than firstname.lastname@example.org. But, nevertheless, what are your thoughts about that?
This is one area where a small startup can do a much better job than a big corporation so try and take advantage of it. Atlassian, which is not a small company, prides itself for its "legendary support." http://confluence.atlassian.com/display/Support/How+to+Get+Legendary+Support+from+Atlassian If you can do the same, it's one way to show that you're much better than the competition. Your customers than say "wow, these guys really care about their customers."
Now to go to your original question, it really depends on the nature of the email and the service you're offering. I tend to say that all email from customers should be replied to within one business day. If you need more time, you should reply and indicate that "I'm researching your question and I'll get back to you in xxx days."
But if you're offering web hosting and an email comes in from a customer that their website is down, it should probably be answered in 1-2 hours.
Also, you should be using a ticket system, like zendesk rather than a regular email account, that way multiple people can watch the queue.
The #1 rule is to tell people what to expect (eg in a contract, website, advert), and then deliver a better than expected service.
The level of service really depends on the norms for your markets. See what the norm is and decide if you want to match it, improve on it, or stay below it.
Aside from that, if you are starting up as one man band, bear in mind that it isn't always best to answer sales emails immediately, as this can give the impression you have nothing to do!