You don't get a TS at ROTC, and very rarely on active duty unless your job requires it. The few pilot jobs that require a TS: most fighters, nuke transport, all AFSOC, some -135 AFSOC tankers, certain DV units. You'll get a secret in ROTC and it will last for 10 years, then they re-read your file and update it for another 10. If you need the TS, there is an assload of paperwork to do, and they interview a bunch of your family and friends. I think it expires every 2 years, and they have to re-certify it.