Security clearances should be revoked the minute they are not longer needed. Any government employee who leaves a government position where security clearance was necessary should lose that clearance when leaving the government job. The idea that people keep their access to classified information even after they leave their jobs is the silly. That goes from elected officials as well. There is no reason for example that Hillary should have kept her clearance after leaving her job as secretary of state, or Brennan after leaving his role as the head of the CIA. If millions of people have security clearance then what does it mean to have security clearance?
And of course, if you violate security policy then your clearance should be removed as well.