From our friends at the Democratic Party of Milwaukee County:
Early in-person voting begins in the City of Milwaukee on Monday, October 22nd
If you live in the City of Milwaukee, you can vote early starting this Monday. Hours are:
Monday through Friday, 8:30 AM to 7:00 PM
Saturday and Sunday, 9:00 AM to 5:00 PMCast your vote in the Milwaukee Municipal Building, next door to City Hall.
If you’re a registered City of Milwaukee voter, enter on the Market Street side.
ADDRESS: 830 N. Market StreetIf you live in Milwaukee and need to register as a voter, or if you’re a registered Milwaukee voter and need to change your registration, enter on the Broadway Street side.
ADDRESS: 841 N. Broadway StreetIf you drive, you can take advantage of the city parking rate of $1.25/hour in the Milwaukee Center parking structure on the southwest corner of Water & Kilbourn. Just get your parking ticket validated by the Election Commission staff.

How long does the early voting last ? when does it end ?
Not a 100% sure…but in general early voting ends the Friday before the actual election.