Security clearances should normally be suspended or cancelled when a government employee leaves govt employment and no longer needs access to our defense secrets. This is basic and should not require or be limited to , imho, an obviously anti- American security risk like Brennan (who patently should never have even been considered for a security clearance or a government job in the first place )
It doesn’t quite work that way.
Access should be denied immediately when you lose a job. However, your clearance is kept active for about 2 years. (I had a top secret clearance, and I can’t rememberhow long I had it it after I quit working).
It used to take about 3 months to get a secret clearance and a tear to get a top secret clearance due to all of the background checks. Keeping clearances active means access can immediately be given when starting a new job.
With that said, the president has executive privilege to strip clearances to whoever he wants.