The answer to this depends on which version of ISE you are currently using. Assuming you have either subscription plus and apex licenses today and are on 2.2 or newer then this applies.
4.2 License enforcement
Starting with version 2.2, ISE will enforce licenses when they are out of compliance for 45 days out of 60 consecutive days. A license is out of compliance when (a) term licenses have expired; or (b) more sessions are authorized to establish network sessions or use services than are entitled.
For term licenses, alerts will be provided 30 days prior to expiry on a daily basis to the administrator. When the term licenses expire, they are considered to be out of compliance. ISE will provide a grace period of 45 consecutive days, after which enforcement will begin.
When 25 percent more sessions are authorized than are entitled by the license, the license is considered out of compliance (the 25 percent overage is provided to account for a temporary burst). If the license is out of compliance for 45 consecutive days, enforcement will kick in.
Alerts will be provided every day that a license is out of compliance. When enforcement kicks in, there will be no impact to end users. However, visibility and management of the features associated with an out-of-compliance license will be affected.
Enforcement will be for Plus and Apex licenses. These enforcement actions and impacts on licenses may change in the future.
Source:
https://www.cisco.com/c/dam/en/us/products/collateral/security/identity-services-engine/guide_c07-656177.pdf
