Apprentice licenses are valid for 30 days. You will need to install new licenses once 30 days have passed. From H19, you can reactive Houdini Apprentice just by the File->Activate Apprentice menu in License Administrator.

By starting Houdini for the first time or without an active license, you will be prompted with various options:

• Choose the first option: Install (or reinstall) my free Houdini Apprentice license and click Next.
  

• You will be prompted to accept the Side Effects Software License Agreement to continue.
  

Important:

While Apprentice users are able to log into the License Administrator, non-commercial Apprentice licenses cannot be installed here.
Installing licenses through License Administrator is reserved for Indie and Commercial users only.