I have NEVER had Windows (on a machine that has no internet access) send me an error that it needs to be connected to the internet to "re-validate" it's authenticity, AFTER being originally registered and activated, or it would no longer be useable. Once and done. And Microsoft is a large enough company, that their "Phone activation" system will more than likely, always be available.
It can happen, and Shane is correct. The check happens at every boot up. Windows phones home and reports that it is a valid install when it is connected to the net. Check your Event log.
This is in the Applications log and is reported in the Security-SPP:
For the OS:
The Software Protection service has completed licensing status check.
Application Id=0ff1ce15-a989-479d-af46-f275c6370663
Licensing Status=
1: 41499869-4103-4d3b-9da6-d07df41b6e39, 1, 1 [(0 [0x00000000, 1, 0], [(?)( 1 0x00000000)(?)( 2 0x00000000 3 0 msft:rm/algorithm/hwid/4.0 0x00000000 0)(?)(?)(?)(?)])(1 )(2 )]
For MS Office 2013:
The Software Protection service has completed licensing status check.
Application Id=0ff1ce15-a989-479d-af46-f275c6370663
Licensing Status=
1: 064383fa-1538-491c-859b-0ecab169a0ab, 1, 0 [(0 [0xC004F014, 0, 0], [(?)(?)(?)(?)(?)(?)(?)(?)])(1 )(2 )]
2: 090506fc-50f8-4c00-b8c7-91982a2a7c99, 1, 0 [(0 [0xC004F014, 0, 0], [(?)(?)(?)(?)(?)(?)(?)(?)])(1 )(2 )]
3: 0bc1dae4-6158-4a1c-a893-807665b934b2, 1, 0 [(0 [0xC004F014, 0, 0], [(?)(?)(?)(?)(?)(?)(?)(?)])(1 )(2 )]
Etc... There are about 30 of them.
(These aren't license keys - they are GUIDs for the applications).
I had something go wrong on my Alienware which made Windows invalid. I had to reauthorize the OS otherwise I'd be locked out. This was an issue for over a year that I ended up doing a reinstall to get rid of. No matter what I tried, using various techniques from Microsoft and other tech sites, did not resolve the problem.
John