The LAN service will attempt to connect to the Entitlements server on a daily basis. If it cannot connect it will put the Entitlements into Offline mode. At this point there are two controlling factors on how long TFS can be used for.

Number of uses. Each catalog item has a defined number of uses that can be made in offline mode.
Grace days.

When you run TFS and the entitlements are offline you will see a message advising how many runs and grace days are left.

If you log into the LAN service you will be taken straight to the View Entitlements page and that is the only option available.

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