There exists an API to create a report about the duration of completed process instances:

AT:

• HistoryService#createHistoricProcessInstanceReport() exists
• possible filter options are: startedBefore, startedAfter, processDefinitionIdIn, and processDefinitionKeyIn
• when executing the report it is possible to set a period unit to define the report granularity:

  historyService
    .createHistoricProcessInstanceReport()
    . ...
    .duration(MONTH);
  

• if authorization is enabled, the following authorization is needed:
  Permission: READ_HISTORY, Resource: PROCESS_DEFINITION, ResourceId: *
• corresponding REST API exists