Hi:
I was recently given an outstanding white paper called "Migrating PST Files into Symantec Enterprise Vault 2007" by Andy Joyce, Senior Field Enablement Manager, Enterprise Vault, Symantec. It doesn't seem to have an official part number, but if you E-mail me I'll send it to you.
In any event, we have found that it varies somewhat, but this document indicates performance of up to 2 GB/hour can be expected. In the published performance guides there are also figures around 1.5GB/hour.
Hope this helps ... Mark