If you are asking about Florida State prison sentences then the answer is that if you behave in prison, and if you are not serving a minimum-mandatory sentence, then you will serve 85%.
If you are asking about any State other than Florida then you should repost your question with a different location.
Unlike the FL-DOC, local jails operate on municipal rules and are not standardized throughout the State. You can find out the gain time for any particular institution (state, local or even federal) by doing a Google search for the specific facility and either reading the information online or contacting them directly and asking.
For example, the Ft. Pierce jail's website is: http://www.stluciesheriff.com/disclaimer.php
I hope that I have been helpful in answering your question.
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The answer is found in the Florida Statues for Florida and each state has their own laws as well as the Federal system. For the rest of the answer, see Mr. Haber's post which is spot on correct, as usual.
The information provided is not legal advice from Criminal Defense Lawyer Albert Quirantes, or the Ticket Law Center in Miami, Florida. There is no attorney client privilege created in this communication. Do not send questions which are confidential in nature by either this venue or via email. Personal questions should be asked in person or via telephonic conference only. You should only ask theoretical questions of a general nature.
It is 85% in FL. You can go to the FL department of corrections website to find all sorts of useful information, even a rather telling history of corrections in FL.
R. Jason de Groot, Esq. We do not have an attorney-client relationship. I am not your lawyer. The statements I make do not constitute legal advice. Any statements made by me are based upon the limited facts you have presented, and under the premise that you will consult with a local attorney. This is not an attempt to solicit business. This disclaimer is in addition to any disclaimers that this website has made. I am only licensed in Florida.