Register or Login with your VMware account credentials
Once you have logged in , You will have been provided with the option to download the ESXi 5.5 Installation ISO and VMware vSphere client along with the Free license key for ESXi 5.5 Hypervisor.
Once the installers are download. Complete the ESXi 5.5 installation or Upgrade and Connect to your ESXi host using vSphere Client.
Click on Configuration tab and select Licensed Features. Click on Edit
Enter the license key and Click on Ok.
You are done with the ESXi 5.5 free license key assignment and it is never Expires now. I believe this is informative for you. Thanks for Reading !!!