Half a day
Mezquita at opening, Patio de los Naranjos, then out through the Judería to the Roman bridge. That is the city's outline and most of it is free.
One day
The half-day above, then the Alcázar gardens in the late afternoon when the light is low. Eat around the Corredera rather than near the Mezquita.
⚠️ From June to September the middle of the day is genuinely hostile. Plan to be indoors or under trees between roughly midday and late afternoon — it is the single most useful thing to know about visiting Córdoba in summer.
Two days
Add Medina Azahara, which needs half a day of its own, and Palacio de Viana for the patios.
Three
Add the archaeological museum and the Roman layer — the temple and the theatre under the museum — then a trip out of the city.
