When the period comes to an end, run slmgr -rearm to extend it by another 180 days.
Use Powershell and run:
ps> slmgr -rearm
Next restart your computer.
ps> Restart-Computer
Once restarted, open PowerShell and check your settings.
ps> slmgr -dli
You can do this up to 5 times (i.e. 3 years)