The "hosting backup garbage collection" module is an Aegir extension which allows you to specify how long Aegir should retain backups for. You can specify, for example, that after a month, only one backup a week should be kept. You can specify as many of these rules as you want, so you could keep hourly backups for a day, daily backups for two weeks and monthly backups thereafter.
Currently it applies to all sites hosted by Aegir, and there is no way to exclude specific sites.
Install as any other Drupal module into your hostmaster site.
Enable the feature in the 'experimental' section of 'admin/hosting' page.
Go to: admin/hosting/backup_gc