Wis. Stat. § 411.205: Firm offers.

An offer by a merchant to lease goods to or from another person in a signed writing that by its terms gives assurance that it will be held open is not revocable, for lack of consideration, during the time stated or, if no time is stated, for a reasonable time, but in no event may the period of irrevocability exceed 3 months. A term of assurance on a form supplied by the offeree shall be signed separately by the offeror.