Sentence reduction comes from good behavior time credit, residential drug program (federal) or cooperation with the government. If you are cooperating you can get the credit before or during your sentence - there is no prerequisite of time served.
https://www.inmateaid.com/ask-the-inmate/who-is-eligible-for-a-sentence-reduction-and-how-much-of-your-time-do-you-have-to-serve-before-you-can-apply-to-get-your-sentenced-reduced#answer
Accepted Answer Date Created: March 09,2013