The rules in state prison are 85% of the sentence must be served, unless there is a parole provision in the Judgement and Commitment papers. There is a 15% good time reduction that is granted at the beginning of the sentence - the inmate can only lose that over time, not get more good time.
https://www.inmateaid.com/ask-the-inmate/my-husband-got-sentenced-for-6-mons-in-a-state-jail-in-bartlett-texas-i-was-wondering-if-he-would-be-able-to-get-a-shorter-sentence-with-good-behavior-since-reading-some-of-these-questions-last-i-was-told-is-that-there-is-no-good-time-served-reduction-for#answer
Accepted Answer Date Created: June 19,2015