Set the policy once
Flip on Auto-archive in the toolbar and pick a window: archive any preview branch that hasn't had a deployment in that many days, counting from its creation if it never deployed. The default is 14 days. From then on the cleanup runs on its own, retiring branches as they fall idle. An archived branch turns read-only and stops counting against your active-branch limit, and you can recreate a branch with the same name whenever you need it back.
Keep the branches that should stick around
Long-lived branches like main or staging shouldn't be on a timer. Add them to the keep list and they're never auto-archived, however long they sit idle. The list takes exact branch names, no wildcards, so nothing gets swept up by a loose pattern.
See a cleanup before it runs
Before you save, the Preview section spells out what the policy would do right now: how many branches are eligible to archive immediately, how many will archive later if nothing new deploys, and which kept branches it will skip. You set the rule with the outcome in front of you.
Try it
Open Preview branches, flip on Auto-archive, and set your window. It's live in your dashboard now.