Mich. Comp. Laws § 168.471
Petitions under section 2 of article XII of the state constitution of 1963 proposing an amendment to the constitution must be filed with the secretary of state at least 120 days before the election at which the proposed amendment is to be voted upon. Initiative petitions under section 9 of article II of the state constitution of 1963 must be filed with the secretary of state at least 160 days before the election at which the proposed law would appear on the ballot if the legislature rejects or fails to enact the proposed law. Referendum petitions under section 9 of article II of the state constitution of 1963 must be filed with the secretary of state not more than 90 days following the final adjournment of the legislative session at which the law that is the subject of the referendum was enacted. Not more than 15% of the signatures to be used to determine the validity of a petition described in this section shall be of registered electors from any 1 congressional district. Any signature submitted on a petition above the limit described in this section must not be counted. When filing a petition described in this section with the secretary of state, a person must sort the petition so that the petition signatures are categorized by congressional district. In addition, when filing a petition described in this section with the secretary of state, the person who files the petition must state in writing a good-faith estimate of the number of petition signatures from each congressional district.
MCL 168.471