Normally a new User Group Policy does not take effect until the user logs off and back on, and a Computer Group Policy does not take effect until the computer is restarted. Group Policies are also applied in the background in regular interval.
By default, Group Policy background refreshes occur approximately every 90 minutes with a 30 minute offset. By default, Domain Controllers refresh their policies every 5 minutes, while member servers and desktops refresh their policies every 90 to 120 minutes. This automatic background refresh is controlled by a GPO setting under the "Computer Configuration > Administrative Templates > System > Group Policy" node. The Group Policy setting you have to select is "Group Policy refresh interval for computers".
If you want to apply a new group policy immediately, you can do so using the "gpupdate" utility. "gpupdate" utility can be executed from cmd.exe. You can specify User or Computer-side policies with the /target switch (gpupdate /target:user or gpupdate /target:computer).
The "gpupdate" utility normally only downloads and applies Group Policy settings that have changed since the last download for conserving bandwith. If you want to download and apply all policies, use the /force switch (gpupdate /force).