Buenos Aires  deserves all the time you can give it. Those planning an extended stay, say two weeks or more, should acquire Moon Buenos Aires , which has exhaustive coverage of the capital and more detail on excursions in Buenos Aires Province and neighboring Uruguay.
At a minimum, though, figure three days for highlights like the Plaza de Mayo , adjacent sights like the presidential palace  and cathedral , and the Avenida de Mayo and Café Tortoni ; the southern barrios of San Telmo  (preferably on a Sunday) and La Boca ; and the Teatro Colón , Recoleta  and its famous cemetery , and Palermo  (including the MALBA  art museum).
But that’s pressing it, and at least a week would be desirable for seeing those sights thoroughly, at a more leisurely pace, and taking in the Puerto Madero  waterfront, Palermo ’s parks and fine arts museums, Chacarita cemetery , and the Paraná Delta .
Even that would permit only limited partaking of the rapidly evolving restaurant scene in Palermo Soho, Palermo Hollywood, and Las Cañitas; the vibrant cultural life (including art galleries and theater); the hyperactive nightlife; and even compulsive shopping at Galerías Pacífico  and other recycled landmarks.
No matter how much time you spend here, there’s always more to see and do.