Group policy refresh interval

Hi all,

I have windows 2003 DC and I have more than 100 client m/c.I applied a new policy for clients for proxy setting.first time when user log on it applied automatically.I changed the ip from client and given new ip.I restarted the machine and checked.but the Group policy not applied.The changes i have made remain same in proxy setting.
Please help me to resolve this problem.
Note:If i run gpupdate in client machine the group policy is updating.