It all depends on what sort of additional information they are looking for. It also depends on your personal status (i.e. have you been out of status ever, etc.).
It is a fairly common scenario these days, so I wouldn't worry if I were you. Just make sure you have a competent lawyer.
My girlfriend got an RFE from Vermont about her job title and requirements, and after the lawyer filed the response, it took her about a month to get back the approval.
