Hi,
you can easily see the usage data in the Management Console. There are searches behind the panels (little search icon in the lower right of each panel). The data for the searches is in your Splunk Environment. You can now take and modify those searches to time bucket by hour and/or month. It would make sense to save this search as a report, let the report run regularly (scheduled) and use a summary index for the results. After that you can use the REST API on your Splunk SearchHead to trigger a search against the summarised data and get the results as a csv or json.
I can't give you a specific search cause that depends on your license model and Splunk version. But the panels in the Management Console Dashboards give you the searches you need to start from.
Hey I registered myself with the Splunk free trail but I was not able to get to the usage data management console, this is what I have landed up to, does any of the following include the actual host apart from the license tier the customer belongs to?