A. In a county with a population of two hundred thousand persons or more a grand jury shall be called every four months each year by the presiding judge of the superior court. Each grand jury shall sit until a new grand jury has been impanelled to replace it or until its term expires pursuant to section 21-403. Additional grand juries shall be called on petition of the county attorney stating the reasons therefor.

B. In a county with a population of less than two hundred thousand persons the presiding judge of the superior court may call a grand jury either:

1. If, in the judge's sole discretion, the judge determines the public interest requires a grand jury.

2. On petition of the county attorney.

. . .

A. Grand juries called pursuant to section 21-402 shall serve for a term designated by the presiding judge of the superior court which shall not exceed either:

1. One hundred twenty days if the grand jury is called pursuant to section 21-402, subsection A.

2. One hundred eighty days if the grand jury is called pursuant to section 21-402, subsection B.

B. If at the end of such period the grand jury is serving in connection with unfinished inquiries or investigations, the presiding judge may extend the term , upon petition by the county attorney stating the reasons therefor, until the conclusion of the investigation.

A.R.S. § 21-403