Hi David,
With proper configuration (syslogs are sent from CUCM to PCA), PCA can detect an endpoint getting registered/unregistered in roughly 10 minutes. Then, for the graphs to be refreshed, it takes about 20 minutes. I would say that's about the correct time you are seeing. If you are looking for (near-)real-time statistics, i'm afraid that's not possible yet.
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NMS Team
Krakow, Poland
Cisco TAC