For vSphere 5.5 - replication happens every 30 seconds automatically.
For vSphere 5.1 - replication is manual via the export/import tool (repl_tool.cmd)

For more information on how to do the replication in vsphere 5.1, VMware has an excellent KB article:
http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=2038677