Each and every steam corporation holding consumer deposits shall render to each depositor, when his deposit is refunded, a statement showing the amount of the deposit and the date thereof; the amount of unpaid interest accrued thereon to date of the refund and the period covered thereby; and the amount of deposit and interest refunded. Each depositor upon becoming entitled thereto, either by reason of nondelinquency for one year or upon ceasing to be a consumer, may withdraw his deposit and all interest thereon which has not been refunded or credited against bills for service upon surrendering his deposit certificate (or submitting satisfactory proof of ownership of the deposit or of the right to receive the same) and the payment of all bills for which Such deposit is security. The “prudent” period of deposit retention, referred to in Public Service Law, section 117, may not exceed two years of nondelinquency in the payment of bills rendered.