Automatic billing? No, CBSubs is not a bookkeeping extension or anything of the sort. Users are charged at time of renewal and are not sent a bill, but an invoice for their purchase (again, not bookkeeping). Custom subscription durations are currently not supported, but can be done using some complex database queries with CBSubs SQL Actions. Example of such usage can be seen in the below thread.
www.joomlapolis.com/component/option,com_joomlaboard/Itemid,38/func,view/id,97062/catid,90/