How often are clearances reviewed and how long are they good for?

Generally, as long as cleared individuals remain employed by a cleared contractor or government agency and are reasonably expected to require access to classified information, their personnel security clearance will remain in effect, provided they comply with Periodic Reinvestigation requirements.  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. As of 2021, DISA has successfully enrolled all DoD Service members, civilians and contractors with a security clearance - about 3.6M in a continuous vetting program, which will ultimately replace these periodic reinvestigations. The goal is that DoD personnel who do not have a security clearance will also be included in this continuous vetting under its trusted workforce, TW2.0 goal.