Every disability insurance policy shall contain clauses providing for a grace period of at least 7 days for weekly premium policies, 10 days for monthly premium policies and 31 days for all other policies, for each premium after the first, during which the policy shall continue in force. In group and blanket policies the policy must provide for a grace period of at least 31 days unless the policyholder gives written notice of discontinuance prior to the date of discontinuance and in accordance with the policy terms. In group or blanket policies, the policy may provide for payment of a proportional premium for the period the policy is in effect during the grace period under this section.