At each public hearing upon a marketing order or a major amendment to it, the director shall receive evidence relative to the period of time which may be necessarily required for the filing, checking, and tabulating of written assents which are prescribed in Article 12 (commencing with Section 58991) of this chapter. He shall also, at the conclusion of each such hearing, make a finding, based upon the evidence, with respect to the period of time which may be reasonably and necessarily required for such filing, checking, and tabulating. On the basis of such finding, the director shall fix and limit the period during which such assents may be received by him and shall be the period at the time of mailing of assent forms to producers and handlers that are affected. If, however, the director finds that an extension of such period is reasonably warranted, he may extend such period for receiving assents.