The rule in plain English
Wisconsin gives 21 days, but from one of three dates: the day the rental agreement ended, if the tenant left that day; the day it ends, or the day a new tenancy began, if the tenant left early; or the day the landlord learned the tenant had gone, if the tenant left late (Wis. Stat. 704.28(4)).
The administrative code repeats the rule and adds the written statement of every amount withheld (Wis. Admin. Code ATCP 134.06).
For a tenant who is behind on rent, the Wisconsin notice is available here.
Prepare the Wisconsin noticeMore for Wisconsin landlords: Wisconsin Eviction Notice Selector · See the Wisconsin documents