Upvote:5
For what it's worth (probably not much), Google Maps gives a walking route from Santiago to Buenos Aires as follows:
Total distance is 1,575 km, or about 319 hours of walking.
Also, the above link notes that the pass at Paso del Planchón is only open from November 1 to April 30, i.e., the summer. Crossing the Andes by foot in the winter would, of course, be a Bad Idea, and even in the summer it'd be dicey. Some pictures of the route up to Paso del Planchón are available here; that page also notes that snow can come on suddenly, and that the nights are quite cold.