Updates to the AD schema replicate to other DCs the same way as any other update. If the servers are in the same AD site the replication will begin within 5 minutes of the change. If the servers are in different AD sites then the replication schedule on the site link will determine the duration of the delay before tart of repliation.
I'd recommend installing "REPLMON" from the "Windows Support Tools" (in the "\SUPPORT" folder of the Windows Server CD) so that you can get a graphical view of the replication topology and the last replication status for each partition of the AD. You can force replication from REPLMON the same as you can from AD Sites and Services, but you get more feedback from REPLMON.
The USNs you're looking at don't have to do with replication of the schema. Those are the update sequence numbers on the given objects you're looking at. You want to look at the replication status of the Schema / Configuration partition itself.