AppFabric WCF statistics share many things with WCF performance counters & WCF analytic tracing.
Unfortunately, when working with async operation call duration, call duration is simply not supported because of the worker/IO thread switch.
You can find here and here more informations
When used on an asynchronous WCF service the Call Duration counter will always return -1.
You can try to emit user-defined events to track operations duration, but this will be be visible in the dashboard.