I would like to get your opinion on what would be a good length after which an Jira or Confluence staging instance would be needed to be refreshed? I would understand that factors like size of the instance and the amount of changes may affect this. Let me know what do you'll keep in mind when doing this and how would you approach this issue.
At a minimum this should be before any upgrade so your upgrade testing on Staging is as similar to Production as possible.
Otherwise it depends on whether any configuration is allowed to skip the Staging environment and go straight to Prod (common with set up of templates or adding/editing location lists).