Further observations using DU meter for the past hour. The PC is just running RB and an instance of Outlook Web Access occasionally polling my work mailbox.
Hourly total is 18.6MB with approx 1100 network flights at present
Extrapolate for the day: 446MB
For the week: 3.1GB
And the month: 12.5GB
It's worth noting that as the number of network flights drops off considerably during the late evening and wee, small hours you could treat that estimate as worst case scenario.