Businesses come and go on the web, and only those who show true interest and enthusiasm about what they're selling will survive. Or they may request a free sample. And all of this will most likely be done through e-mail. Answer your e-mail, and answer it promptly. It gives people information in a much shorter amount of time than having to do through a more traditional route of finding what they want.
Customers expect FAST replies.
An appropriate response time in my opinion should be 48 hours. If you wait a month to answer a request from a customer - forget it. Within 24 hours. Nothing irritates a consumer more than to order from you, receive a product with a problem, then have to wait over a week for your reply as to how they should handle the situation. If you wait longer than one day to respond to a customer with a complaint, you might as well kiss future sales to that person goodbye.