Change replication schedule between DC

It loks as though the default is 5 minutes. Check this article:
http://support.microsoft.com/support/kb/articles/Q214/6/78.ASP

To modify the delay between the change to the Active Directory and first replication partner notification, use Registry Editor to modify value data for the "Replicator notify pause after modify (secs)" DWORD value in the following registry key:
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\NTDS\Parameters

The default value data for the "Replicator notify pause after modify (secs)" DWORD value is 0x12c, which in hexadecimal format is 300 decimal (5 minutes).

The 15 minute limit indicates that you're talking inter-site replication. Sites are used to limit replication between DC's that are connected by a slow link. MS suggests that a 'slow link' is less than 512 Kbps. If your link is faster, put the DC's in the same site and then 5-minute replication should indeed be the default.