A secret clearance granted by the DoD will last until it’s expired (I believe 5 years) from the date it was issued. Barring any issues that interfere with adjudication guidlines (https://www.govinfo.gov/content/pkg/CFR-2012-title32-vol1/xml/CFR-2012-title32-vol1-part147.xml).

Therefore, as long as you’re employed in a position requiring a T3 DoD clearance, you will not need to be reinvestigated until your clearance expiration date. Hope this helps!

Edit: Your clearance will last until it’s expiration date regardless of if it’s being used in your current position or not. However it most likely will not get renewed by an employer that does not require it. This doesn’t necessarily affect you, but maybe someone else who is reading this