Córdoba is small, so the answer depends on how much you want to add to the essentials.
One day
Enough for the Mezquita, the old town and a walk by the river, if you start early.
Two days
Room to add the Alcázar and Medina Azahara, and an unhurried evening.
Three days
Time for patios, a baths visit and a day trip to a nearby village.
Keep exploring
