The license keys are perpetual for the major version of the product the license is issued (v2021 currently).
Software Assurance Subscription
Subscription is an annual upgrade protection fee that provides free new major version upgrades. Subscription is the only way to get the upgrades at this point.
New major version licenses
Each new major version requires a new license code. The licenses are automatically issued to the accounts when the latest version is released as long as the account's upgrade protection is current.
page if you need to find the new version license key, lost your license key information, or want to verify the upgrade protection expiration date.
If you placed original order without the upgrade protection or the subscription expired, purchasing new licenses is the way to use the latest version of the product.