However, publishing is a peculiar business. This is not an apology for that, and some may argue that this is no excuse, that if editors are unable to operate in ordinary businesslike fashion, but in more of a hobby-like fashion, then they ought not to have or need exclusive access to your product beyond a certain amount of time. There the editor may say they take a month, or two, or six. If you're within their stated limit, don't panic. Two months is probably the minimum you should ever wait before becoming even the slightest bit antsy, even with an ordinarily fast market.
Some folks suggest nothing happens in the novel market in less than three months.
Yet another rule of thumb is to wait twice the guideline-stated or average response time. Still another metric is to wait until you've exceeded the maximum from whatever response time lists you read, such as the Black Holes web page, or the others mentioned thereon. Okay, it's been too long. Once you've decided it's been too darn long, the most common action is to send the editor a query. For that matter, since it bears repeating, the same holds true if you violate any of the other standard submission rules, like not using standard manuscript format or ignoring any personal preferences the editor sets forth in their guidelines.