Wis. Stat. § 67.07
The principal of every sum borrowed and secured by an issue of municipal bonds may be made payable at one time in a single payment or at several times in 2 or more installments; but every installment, whether of principal or interest, shall be made payable not later than 20 years after the original date of the bonds, except that when the bonds are issued for the acquisition of lands by a county having a population of 150,000 or over, for public, municipal purposes or for the permanent improvement thereof, by a sewerage district or county having a population of 150,000 or over for sewerage purposes and by any city for sewerage purposes, all installments of principal and interest shall be made payable within a period not exceeding 50 years from the original date of the bonds. The terms of the bonds shall comply with the initial resolution.
Wis. Stat. § 67.07