If you set it to 30 days , it will be deleted 30 days after the group was created if not renewed! NOT when you activated the policy! All groups that is older than whatever the policy states, will get 30 days to be renewed. However, the group gets soft-deleted meaning it will be in the recycle bin for 30 days. Also all retention policys apply! The least amount of days is 30 days so I guess 0 will do nothing. Not sure about if it means unlimited. Why configure it on the group then..