@MGMullen - we archive on an annual basis.  In Portfolios it is a time consuming process, thus why we only perform once a year.  We worked with key stakeholders to identify how far back data should be maintained, which was about any inception to date reporting needs. So I think every company could be different depending on business rules. We do this to move out projects that are no longer needed for financial reporting and thus can be removed.  

Note; You cannot report on items in the archive database so be cautious on how much and how frequent you archive. There is an ability to restore from the archive, which is nice, but you want that to be an exception and not a common practice.