Go to VMware.com => products=> choose ESXi=> cloick on Free Download Now => login=> then you see the license, this license is just for you and registered just for you.
Then go to ESXi Host with vSphere Client, Configuration Tab of host, Licensed Features, then edit and add the License, then you see the features become less but the license never expires.
If it is helpful, would u please mark it as "Helpful" or "Correct"?
Thank u
Saman