I first booked Alfonso's place for three months, but I liked it (and the city in general) so much that I extended the reservation to six months. That is basically all you need to know.
The apartment has everything you could possibly want (including washer/dryer), and the area is IMO perfect. It is technically Zona Rosa, but in a more residential part of the neighborhood away from the "strip". The street outside is quiet, though the apartment faces away from the street so that the apartment itself is even more quiet. It's also a five minute walk to the Sevilla metro station, there are multiple places to eat/shop/exercise nearby, etc. In terms of security, I never felt unsafe, and I would sometimes walk home from places very late at night.
Alfonso and his cousin Salvador (who helps with taking care of the apartment) are both very friendly and extremely reachable whenever you have any kind of question or issue, either about the apartment or about the city in general.
This is probably one of the best Airbnb locations I have stayed at.