If you are staying in towns you will not need a car - Sevilla, Granada, Cordoba, Ronda, Malaga, the beach resort towns, and plenty of others - all are connected by train or bus, and when you are in town you don't need a car. You can often walk to the main sights and neighborhoods if your hotel is central, and city busses and taxis are abundant. In fact, driving in these towns isn't a good idea - there are tiny streets, limited parking, and restricted zones. If you have a car because you will need it at other points in your trip, you would be best to park it outside of the old town, in a garage, while you visit one of the above mentioned towns, and pick it up when you leave.
Where a car would be handy is if you want to string together day trips of smaller towns or villages. Some of the white hill towns near Ronda would be easier with a car, as would the Sierra Nevada mountain towns. But generally people stick to the towns mentioned above and don't need a car.
Sevilla - at least 2-3 full days (so 3-4 nights).