If you are meaning a Certificate of Competence for powder actuated tools the expiry date has to be included on the certificate (and be recorded in the register).
Worksafe may specify the expiration period for the certificates, but I could not find any information that they (or OSH / DoL) have done so for PATs - even the ACoP just mentions the regulator can set when the certificates expire but doesn't include any specific period.
Just for reference, my Ramset PAT CoC expires in 2028, and given I got it in 2005 that's 25 years!

Worksafe may specify the expiration period for the certificates, but I could not find any information that they (or OSH / DoL) have done so for PATs - even the ACoP just mentions the regulator can set when the certificates expire but doesn't include any specific period. — MattD2

Yeah same here Matt, Hilti says that their Operator Cert doesn't expire for tools listed, but may need to be redone to add new tools to it.