A machine credential is valid for 2 years. The ATO has extensive information on their website regarding the requirements for installing a machine credential.
You may required additional steps if the words Software Required still displays in your browser after downloading, installing and enabling your browser extension.
Read more about Saving the Machine Credential keystore file if you use HandiSoft Online (On Access Evo).
Important: If you have recently updated your Tax Agent Details or ABN, all users will require a new machine credential.