15' in my state.
Depending on the bylaws of a municipality, parking is not allowed within 3 to 5 m or 10 to 15 ft from a fire hydrant.
20ft It is actually 15ft in CA. See link for details: http://www.dmv.ca.gov/pubs/vctop/d11/vc22514.htm
15 ft, I think.
Considering that Wisconsin is 320 miles at its maximum, that would be the furthest you could park away from a fire hydrant in Wisconsin and still be in the state.