The refresh interval is 90 minutes plus a random offset of up to 30 minutes. GPOs on Domain Controllers have a refresh interval of five minutes with no offset by default. There are some exceptions however. GPOs that involve folder redirection settings, disk quotas and drive maps only refresh at startup or logon. If multiple sites are involved then you must take into account the AD replication interval which can be another 30 minutes. Jeremy discusses refresh intervals in his book.