According to Princeton University, Plan B only protects against pregnancy for 72 hours before and a few hours after taking it. However, the university does say that no one has studied how many hours of protection it provides after taking it.
WebMD reports that Plan B works primarily by preventing or delaying ovulation if the woman happens to be ovulating around the time the drug is taken. The actual effect on the woman's cycle depends on where in the menstrual cycle she is when she takes the drug. If she isn't ready to ovulate when the drug is taken, it may have no impact at all on her cycle that month.