Currently billing for Azure Security Center is reported on a per-node, per-month basis. Starting July 1st 2018 this reporting will be changed to per-node, per-hour to achieve more granularity in billing. Billing is still pro-rated thus you'll only pay for the time a node was actually using the service.
This billing update comes with a change to the resource GUIDs and if you use them in your own billing routines you need to make changes accordingly.
For more information see the pricing update page.