The ClearanceJobs site says, (among other things):
"Currently reinvestigations are required at 5-year intervals for Top Secret clearances, 10-year intervals for Secret clearances, and 15-year intervals for Confidential clearances. In December 2012 new Federal Investigative Standards were approved and are being phased in very gradually. "
OK, I had my PSI in January 2009 for a Secret clearance (for the DOS); I received the full clearance by the end of the month. It was active until July 2011, then it expired in July 2013 (24 months after). So, if a prospective employer wants to sponsor me for a clearance, does that mean it will be (relatively) faster for me to get my interim Secret clearance, as I do not need another PSI?