With Group Policies (and also Group Policy Preferences), settings are divided into two categories: Computer Configuration and User Configuration. Computer Configuration settings are applied during startup of the operating system. User Configuration settings are applied during user logon on to a computer.
Once policy settings are applied at startup/logon, the settings are refreshed automatically to ensure they are up-to-date. During Group Policy refresh, the client computer contacts the closest available domain controller which provides a list of all the policy objects that apply to the computer and user at different levels (site, domain, OUs). The domain controller does this regardless of whether the version numbers on all the summarized policy objects have changed. By default, the computer processes the policy objects only if the version number of at least one of the policy objects has changed. If any one of the related policies has changed, all of the policies have to be processed again because of inheritance and the interdependencies within policies.
You can also force a refresh manually using gpupdate /force. This will process all policies again, even if their version number has not changed.
By default, computer Group Policy is updated in the background every 90 minutes, with a random offset of 0 to 30 minutes. So, minimum refresh cycle is 90 minutes, maximum refresh cycle is 2 hours (120 min) and assures the load is spread when machines/users contact their domain controllers at the refresh cycle. This policy covers both workstations and member servers and excludes domain controllers.
You can specify an update rate from 0 to 64,800 minutes (45 days). If you select 0 minutes, the computer tries to update Group Policy every 7 seconds. However, because updates might interfere with users' work and increase network traffic, very short update intervals are not appropriate for most installations.