The clearance is good for however long it's good for, regardless of your status (active duty, IRR, retired, out all together, etc...). I'm not sure how long each clearance is good for though. I think it's like 10 years for secret and 5 for TS, but don't quote me on that. Anyway, it's good for those 10 years (I think) from the time the investigation was done. A lot of military people make sure their clearances are good for a long time after they get out because there's a lot of VERY good paying jobs out there for people who already have a clearance. You can pay for your own in the civilian world, but I heard that costs like $50K. Smart people let the military do it for them.