A Query on Post-Submission Etiquette

Here's another opportunity to shed some light for doctoral students, if you're so inclined: What is the usual time frame in terms of finding out what's going to happen to a submitted article? My understanding is that it can take months to review and render judgment upon an article; what I don't know is how much time has to elapse before the impatient writer can fire off a polite query as to how the review process is going, and when he or she can expect to hear a verdict. Two months? Three? Six? Or are such inquiries seen as too pushy no matter when they come?