How quickly should you respond to letters from potential customers?

There is a small but proud company that provides services. That is, selling your own time. There are people who need these services. There is the Internet in which the whole thing happens. How to be here? Elementary: a potential client writes an e-mail with a request to the company. And here the most interesting begins.

How and when should the company respond to the client? On the one hand, I want the answer to be intelligible, and the time is minimal .

But, there are some moments:

• If it happens on the Internet, participants in the process may be in different time zones. The client has noon, the company has midnight.
  Sometimes, before you answer, you need to think or consult. (hurry - make people laugh)
  These factors can be minimized by using the mail "answering machine" . To the arrival of the letter, he wrote: "Everything is cool, do not worry, we will answer you soon." But on the other hand, robots - sucks. Not everyone will like the formal attitude. Plus, in fact, such a letter from a robot means nothing.
  
  Or you can somehow preload and somehow solve the problem: The answer to each client within an hour, no more . But is it as good as it sounds?
  ')
  From the company's point of view, there are some factors:
  
  • The average project lasts about two months.
    Not all customers are equally good, they need to be filtered.
    I will explain the second paragraph. Life has shown that customers who need “yesterday” are bad. Better when there is a clear plan with real deadlines and it is respected. These are cool customers. They know what they want and can wait for a response for a while. And let the "hurry-ups" run to other companies.
    
    How to respond to customers? How fast should it be done? How do you do it personally? If you are a client, what do you think about this?