Temporary, probationary and nonpermanent firefighters scheduled to be on duty an average of more than 20 hours per week shall successfully complete a certified basic training program within 18 months of initial appointment. Temporary, probationary and nonpermanent firefighters scheduled to be on duty an average of between 10 and 20 hours per week shall successfully complete a certified basic training program within 24 months of initial appointment. Temporary, probationary and nonpermanent firefighters scheduled to be on duty an average of less than 10 hours per week shall successfully complete a certified basic training program within 36 months of initial appointment. Training may be suspended during periods of nonservice, but all periods of service shall be counted toward the fulfillment of the applicable requirement.