Wis. Stat. § 72.22: Payment.

(1) When payable. Except as provided in s. 72.225, the tax imposed by this chapter shall be paid on the date 9 months after the decedent’s death.

(2) Advance payment. Anyone personally liable for a tax under this chapter may pay an estimated tax before the tax is determined.

(3) Payment. Except as provided in s. 72.225, at the time that the return is filed, the full tax shall be paid. If a prepayment was made, any additional tax shown owing on the return, as filed, shall accompany the return.