If you read more of the concussion information on the Mayo Clinic web site, you will find a lot of it to be out of date. There is no uniform definition of PCS but most consider it to be when symptoms last more than a month or so. Studies show that the concussions that have symptoms beyond the first few days usually start to show improvement at 12-14 days.

As lightrail said, it does not matter what the time limit for PCS is. It does not change how one would get best recovery results. Statistics show that about 15% of diagnosed concussions will have prolonged symptoms that last 6 weeks to a year or longer. This does not take into account the many concussions that are never diagnosed by a doctor.