You can optionally set an expiry date on galleries, which means the gallery will no longer be accessible to your clients once it expires. To change the expiry date of a gallery, click the "Settings" button in the top-right corner of a gallery while editing a gallery.
You can choose a date for the gallery to expire (the default is 7 days), or you can check the "No Expiry" toggle to create a gallery without an expiry date.