It's 30 days.
Documented in the README for the 4.1.x server.

See below.
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*$$1 License counting changes for "in use"                         *
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With this service level the following changes to in use license counting
are introduced.

License Expiration.
  A license feature that has not been used for more than 30 day will be
  expired from the in use license count. This will not change the registered
  licenses, only the count of the in use licenses.
  Libraries in use will not be expired, only client license features.

License actuals update.
  The number of licenses in use will now be updated when the client session
  ends. An audit license is no longer required for the number of in use
licenses
  to get updated.