What is the process of re-activating a security clearance and how long will it typically take?

PV2 Qua Thompson There is no such thing as reactivating your security clearance. As soon as you let it expire or is no longer needed, then you have to do the investigation again. Also, there is no how long it takes, you just have to get with your security manager and start the process ASAP, then is up to you how long you take to fill out the security questionnaire.

used to I was up to the Command... and they can reactivate up to the previous Lever on an interim basis and it takes 30-60 days to get a permeant clearance, and up to 90 for upgrades... or it used to...

Secret clearances are currently good for 10 years, but that is going to change to 5 years in October 2017. If you have been out for less than two years and your last investigation was less than 5 years, then there is a good chance that your clearance can be reactivated with minimal work (most of which will be transparent to you).

Essentially the contractor's FSO will check JCAVs or JPAS to see what the status of you clearance is (active, dormant, or revoked) and determine if they need to submit a new SF 86 and a complete re-investigation or if they can get by with a periodic reassessment to reactivate your clearance.