Ouch, I'm really sore. Should I take a break for a while? How long?
First off, if the soreness is such that you feel painful stabs when you move/stretch the muscles you definitely should rest for awhile (at least several days, maybe up to 6 weeks), and perhaps should see a doc.  This can be a sign of tendinitis, muscle pulls, or a muscle disorder, and continuing to exercise in such conditions can lead to permanent damage.

General soreness/stiffness, on the other hand, is believed to be caused by microscopic muscle injuries that, as they heal, tend to strengthen the muscles.  For such discomfort you need to take is easy for a day or two, until the soreness/stiffness abates.  However, moderate exercise (well below your max effort) to keep yourself limber is often advised.