The configuration eventstream.daystokeepevents documented in xwiki.properties allows to:

Number of days the events should be kept (0 or any negative value: infinite duration)
Note: if this value is greater than 0 a scheduler job will be created, this job will then be fired every week to delete events older than the configured value.

However right now this option is only implemented in legacy: it has been moved when the new Solr storage has been put in place for notifications. Meaning that users using XWiki without legacy packages cannot use that feature.