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.