Wis. to Michoacan
All depends of course on which routes you pick but as a rough estimate figure on a week : Madison to the Tex-Mex border about 3 days using interstates mainly.Starting Mexico at any of the crossings between Laredo to Del Rio and you can get to Lazaro Cardenas on the Pacific coast of Mich in three days more if you push it .I know, a few will swear they can do it in two days- but are they enjoying it? The first segment from the border e.g. at Eagle PAss and Mex 57 to Saltillo then Mex 54 to Zacatecas will be a fast run , much wide open plains ,semi desert and rangeland, can do 500+km in a day easily .Mex 71 south through the Bajio to Zamora is more heavily populated agricultural country, much to see ,explore ergo fewer km in a day. Then you are into the mountain country and south of the Bajio and slow down some more for all the curves and scenery to Uruapan and the last stint to the coast on Mex 37 through some hot country of the Rio Balsas valley system and more mountains.
Of course you can pick different roads such as going through Guadalajara -Colima-Tecoman and then east along the coast, or the more time consuming but very scenic and enjoyable routes from the lower Rio Grande valley through Queretaro-Toluca-Ciudad Altamirano then Mex 134 to the coast near Zihuatanejo and west up the coast to Mich.
So a week is about the minimum - but you can drag it out to a very big lot longer. Go slow in Mexico , 300km to 400 km per day is about the max if you figure in time for breakfast, lunch scenic detours.Nothing wrong with a 100km day! And get off the road before dark.