Two days
Enough for the Alcázar, the Cathedral, Santa Cruz and one evening of tapas. Book monument tickets ahead and accept that you will leave wanting more.
Johnny's Seville
Three days is the honest answer for a first visit. Two works if you plan well; four lets the city slow you down, which is rather the point.

Enough for the Alcázar, the Cathedral, Santa Cruz and one evening of tapas. Book monument tickets ahead and accept that you will leave wanting more.
Add Triana, the river and Plaza de España without rushing. You start to eat at Sevillian hours instead of fighting them.
Room for a day trip, a second evening in Triana, or simply sitting in a square until the light changes. This is when the city stops being a list.
Frequent questions

5 hours · The complete Seville experience

3 hours · The food side of Triana, hosted by a local who grew up in it

3 hours · Three remarkable rooms, three signature cocktails, and the stories Seville tells after dark
Tell me how long you are staying and I will suggest where a private experience fits best.