Doug, I think you may be referring to some research done by IPI, I will have to look up the specifics but in a nut shell, toning to completion is necessary for archival protection. Something in the range of 8 minutes comes to mind.

As we are inbetween quarters I will not be back at class until Wed night when I will check on some information and get back to this thread as I don't want to mis lead or mis quote that information.