Except for a prisoner sentenced to a term of life imprisonment without parole, every prisoner confined in any penitentiary, jail or prison for a violation of the Virgin Islands law for a definite term or terms of over 180 days or for the term of his natural life, whose record of conduct shows that he has observed the rules of the institution in which he is confined, upon recommendation of the Director of the Bureau of Corrections supported by the recommendation of a psychiatrist and/or psychologist, may be released on parole after serving one-half of such term or terms or after serving 15 years of a life sentence or of a sentence of 30 years or more or after serving the minimum sentence required by law, whichever is greater; Provided, however, That the Board of Parole, in its discretion by at least a two-thirds affirmative vote of all its members, upon recommendation by the Directors of the Bureau of Corrections, supported by the recommendation of a psychiatrist and/or psychologist, is authorized to fix an earlier eligibility date for the release of a prisoner on parole after serving one-third of his term or terms or after serving 10 years of a life sentence or of a sentence of 30 years or more.