Oh - and I should note that you can retrieve the latest retention report - as well as the retention report for a specific start date...
https://docs.braincloudservers.com/api/capi/report/sysgetlatestreport
What we don't have right now is an API to list the reports that are generated and waiting for retrieval. We are working on that for a future patch.
Also note that we currently keep daily reports for 10 days - and weekly reports for 37 days.