Periodically, Windows 7 and Windows Server 2008 R2 check the hardware configuration of the computer on which the operating system is installed. If the operating system detects that the hardware has changed substantially, reactivation is required. The actual weighting factors and threshold values vary, because these values must keep pace with the constantly evolving computer hardware market. KMS clients focus on hard disk changes to determine the need for reactivation.
Client activations are valid for 180 days—a period is called the activation validity interval. To remain activated, KMS clients must renew their activation by connecting to the KMS host at least once every 180 days. By default, KMS client computers attempt to renew their activation every seven days. After a client’s activation is renewed, the activation validity interval begins again.