As explained to me by my last contracting security officer, if you can "transfer" the entity responsible for your clearance from one company to another, it doesn't really expire. So, if you go from Company A to Company B, it stays active. 1:59 PM it goes off the books, 12:00 AM it goes to the new company. If there's a break, then it kind of goes into "limbo" as she called it. It is tied into "re-investigating" you for the clearance and some financial stuff for the gaining company.
When I left Lockheed in '06 and returned to them in '08, I had to do the full SF-86 stuff. I later saw that my renewal date was tied into my FIRST trip there in 2004. I'm not exactly sure about the "limbo" as she called it, but I liken it to putting a computer into sleep mode. You lose the clearance and access, but it is paused until you need it again.
That's probably a poor explanation.