Earlier this week, Citrix published an interesting article about average bandwidth consumption for different XenDesktop 4.0 remote sessions.
While the purpose of that post was to promote its Branch Repeater technology, which may or may not be interesting for you, the provided graph is valuable as a reference for VDI planning:
The graph comes from a 30-pages paper that describes testing environment and methodology: Performance Assessment and Bandwidth Analysis for Delivering XenDesktop to Branch Offices