For Federal cases, parole has basically been abolished. Most federal criminals will serve 90% of their sentence.
For Texas cases, the answer is it varies depending on the crime and the decision of the Parole Board.
For capital murder: as of September 1, 2005, Texas will have a life without the possibility of parole option for capital murder. The capital murder defendant sentenced to life in prison before September 1, 2005, is parole eligible after serving forty years. The release is not automatic–a full vote from the Board of Parole is required.
For the next group of legislatively designated serious offenses like murder, aggravated kidnapping, aggravated sexual assault, and aggravated robbery, the defendant must generally serve at least half of the actual sentence to be parole eligible. Once again, eligibility does not mean release. Most prisoners are not released when parole eligible.
For other first, second and third degree felonies, the prisoner is parole eligible when calendar time plus good conduct time equals one-fourth of the sentence. Good conduct time is earned by participating in work and self-improvement programs. Good conduct time can be subtracted for disciplinary violations.
For state jail felons, parole eligibility does not apply. State jail felons generally serve every day of their sentence.
Time served for misdemeanors in Texas varies by county. In Harris County, misdemeanor defendants usually get two days credit for one day served. In counties with more crowded jails, a defendant may get three days credit for each day served. In less crowded counties, a defendant may serve every day of the sentence.