Yes, I did mean how long I had to enter the country once the 6 months passed.
Hi snupy, I've been researching this pretty heavily, and I'm 99% sure that the PETS certificate is valid for either a) when the vaccine expires, or b) 4 months, whichever comes first. Here's a quote from the certificate:
"The certificate is valid for intra-Community movements for a
period of four months from the date of issue or until the date
of expiry of the vaccination shown in part IV, whichever is
earlier."
But I believe it's also true that you can "renew" the certificate simply by updating the rabies vaccine.
Hope that helps!
-emma