Answering your business email promptly should be a priority for all business. Not only is e-mail an important communication line with your customers, it is often used by them to gauge that your trustworthy.
Talking about the big businesses, surveys show that the Top-500 fail miserably at answering their business email. Jupiter Communications reported that 42% took more than 5 days to answer a simple question. In the world of Internet, that might as well have been forever. If a customer has to wait that long for an answer, most likely she will have taken their business elsewhere. 35% of companies don't even bother to answer at all. I guess, they just don't like customers ;-) Forrester Research is reporting figures that are similar.
If you really want your customer service to shine, you should consider answering your business email twice a day with a 12 hour interval.
It is even better to check out your direct competition by sending them an e-mail as if you are a potential customer. Send them more than one on several days. Especially check out mondays, fridays and weekends. Track the time it's taking them to answer, and implement a procedure to beat them at the business email game.
OK, I understand that for small businesses, resources are limited. But your stream of business email is most likely to be a lot less than for big guns. And if you check and answer e-mail regularly, numbers of e-mails to answer are usually very easy to handle.