On Windows Server, you get 180 days trial. You can then extend it with this command

slmgr -rearm

To check if you are using KMS and how long you have left run the following command from command line and look out for the “Volume Activation Expiration” line to see how long you have left (if you are on volume activation - this will be listed under description as “KMSCLIENT”)

slmgr.exe /dlv