I asked that question yesterday timmy and the staff at the Uni were not able to confirm any time validity of the vaccine passport , possibly that will come to light after some one has actually been to the General hospital and got one directly.
The Thai certificate of Covid vaccine you get directly after your second jab for now seems to be open ended .. as well as your personal data it shows all the details of date , time location of your vaccines and interestingly there is a blank 3rd column marked booster.
Cant reiterate enough at the last station when getting your cert of vaccine double check all spellings etc before you walk away.
Good luck to all going for their second jabs in the coming days.. go early especially if they are vaccing Sinopharm etc on the same day .
DM