Section 31A(2)(b) in The Prisoners (West Bengal Amendment) Act, 1955.
(b)he has undergone, with remission earned, imprisonment for -(i)in the case of a prisoner sentenced to imprisonment for a term of less than five years, half the period of his sentence, or(ii)in the case of a prisoner sentenced to imprisonment for a term of five years or more, half the period of his sentence or a period of three years, whichever is less;