petri[.]com/how-to-extend-windows-server-evaluation
Windows 7 trial period was 30 days -- Windows Server gives you 180
You can extend -- restart -- a Windows Server trial 5 times, after which it'll have to be reinstalled fresh.
From a command prompt running as admin, you can check trial status with: slmgr – dlv
From a command prompt running as admin you extend the trial with: slmgr -rearm
As noted in the article: "Remember that evaluation copies of Windows are not licensed for commercial use."