This Knowledgebase article provides information about channel bandwidth requirements.
For asynchronous replication, Neverfail requires little bandwidth as it only replicates data deltas. During installation as the two systems are initially synchronized, bandwidth requirements depend on the size of the data store.
How much bandwidth do I need to put Neverfail over a WAN?
By running Neverfail SCOPE over your WAN, you will be able to calculate the optimum bandwidth for your Neverfail implementation.