The easiest way to track usage is to hit the Usage button on the top left-hand side side of the Devices page.
Data on a device's page is updated in near real time as data sessions are completed by the device while the usage reports in the Usage page are summarized nightly and refreshed.
Usage is calculated by adding up all the bytes of each data session your device has used. This information is provided to us directly by our carrier partners. A usage data session consists of the data transmitted from when a device opens up a packet switched communication channel with a cellular tower to when it tears it down.
Usage data can also be exported for further analysis by going to Usage on the Devices page and hitting Export CSV button.
For more detailed usage information please visit this guide.