Group Policies can be applied when a computer boots up, and/or when a user logs in. However, policies are also refreshed automatically according to a predefined schedule. This is called Background Refresh.
Background refresh for non DCs (PCs and Member Servers) is every 90 mins. With a +/- 30 min interval. So the refresh could be 60, 90 or 120 mins. For DCs (Domain Controllers), background refresh is every 5 mins.
Also, every 16 hours every PC will request all group policies to be reapplied (user and machine) These settings can be changed under Computer and User Nodes, Administrative Templates, System, Group Policy.