Security clearance's last one year from the date of last use.
IE: If you separated from the military with an active Top Secret Security clearance, then it will remain inactive for one year after you ETS.
They will just have to update it for the time your clearance was inactive.
Your clearance goes inactive, as soon you you do not use it.
People confuse that with the time a clearance last before you have to have your back ground check updated.
Clearances must be updated at specific intervals, even if they are active clearances.
Top Secret - every 5 years
Secret - every 10 years
Confidential - every 15 years.
Even on active duty, you can have a clearance go inactive, if your present duty position does not require a security clearance.
IE: Recruiter, DI, etc
If that position last more than a year, they do a partial background check again before you can regain your clearance.