Hi crickes,
The automatic deletion of old monitoring indices is available only for exporters of type local and the retention duration can be configured using the setting xpack.monitoring.history.duration if you have a non-basic license (see https://www.elastic.co/guide/en/x-pack/current/monitoring-settings.html#monitoring-collection-settings).
This feature is not available when using http exporters, in this case you can use Curator.