Mich. Comp. Laws § 18.1356

Sec. 356. The balance in the fund shall not exceed 15% of the combined level of general fund-general purpose and school aid fund revenues. If the balance in the fund at the end of a fiscal year exceeds 15% of the actual state general fund-general purpose and school aid fund revenues for that fiscal year, the excess shall be rebated to taxpayers on the individual income tax returns filed following the close of that fiscal year according to a schedule to be established by law.