> This study says live virus has only been confirmed to be seen for at most 9 days. However, (presumably inactive) virus particles can be shed for months afterwards and can show up as PCR positive.

The paper cites 34 replication cycles as the lower bound to obtain viable viral cultures. Whether a PCR test is reported as "positive" depends on the chosen replication cycle threshold (Ct value). As far as I'm aware, this "magic number" is usually chosen to be 35.

Beyond that, it is possible to do more cycles to demonstrate the existence of trace amounts of PCR fragments, but such a test would not be considered "PCR positive".