Sevilla, how long?

Not too long, plenty of day trips. My decision would be based on the overall itinerary.

We spent 4 nights in Seville, and I always wished we had an extra night. Of course, it all depends on your interests. And do you like to have some down time for wandering aimlessly or hanging out at cafes? Seville has many interesting historical and cultural sights. It's also a beautiful city for wandering, with its gorgeous architecture, plazas, fountains, parks, etc? Take a carriage ride; be sure to see a flamenco show. So, I don't think 5 nights is too long for Seville. I think some people visit either Jerez or Cadiz as a daytrip from Seville, but we didn't take any daytrips. There is plenty to do and to enjoy in Seville.

Is 5 nights too long?

No.

5 nights is great, Seville is wonderful. Not sure how much total time you have. There are many great other places to see and if you had a month, you couldn't do the whole country.
Other great places:
Barcelona
Madrid (Toledo and Segovia are nearby and great)
Cordoba
Malaga-Rhonda
Granada (The Alhambra is special)
Valencia
Cadiz
The Basque Region
Salamanca

https://www.travellocal.com/en/destinations/spain

Thanks all. I think we just want to do 2 cities, with 9 nights. 4 nights Barcelona, 5 nights Seville, last night Madrid before home. But I’m open to any suggestions.

Five nights is great for Seville.

I prefer Madrid to Barcelona but that's just me!

I always choose close-together places, because it kills me to waste time moving across the country, but if you think you are unlikely to return anytime soon, then choosing Barcelona and Sevilla works.

Is it possible for you to fly home from Seville? This way you don't have to spend a night in Madrid, and you could add that extra night to Barcelona. When we visited Madrid and Andalucia, we flew into Madrid and we flew home from Seville (with a layover in Lisbon).

For one Spain trip, we divided the time up between Seville and Madrid and used the train trip headed back to Madrid as our "day" trip to Cordoba (storing our bags).

All three of those could be done with less or more time than you have, and 5 nights in Seville and 4 nights in Madrid (or spend a night or two in Cordoba) would be a nice trip.

It also makes logistics a snap. Fly in to Madrid, train to Seville, train to Cordoba and on to Madrid, fly out. Quick train on that route (assuming the system is fully functional again? There was the horrible train crash a little while back).

But Seville also has other side trips that have been mentioned above by others. It's a really nice place to visit.

Barcelona is great too, but I still look at it as a "different" trip - or rather paired with Madrid in a Madrid-Zaragoza-Barcelona grouping.

We spent 5 nights there in November and it was great. We did not take day trips, as there was plenty to do in Seville and we also spent 3 nights in Cordoba so there was no need to go there on a day trip. If we had done a day trip, I would have considered Cadiz. In case it is helpful, my trip report is here:

https://community.ricksteves.com/travel-forum/trip-reports/trip-report-analucia-and-madrid-in-november

There are other helpful trip reports by CWsocial, GeoffB, Carrie, and others if you do a search.

Marsle, why don’t prefer Madrid?

I also prefer Madrid to Barcelona. I’ve visited Barcelona 3 times, each before its current massive over tourism (in 1983, 1988, and 1993), and it just didn’t grab me. No offense to the Catalans, but Catalonia is kind of halfway between France and Spain culturally, and I prefer either of those undiluted. I like museums and art, and Madrid is the clear winner there. It’s actually not as old a city as Barcelona, but it just feels more authentic to me. And Madrid’s day trips are better (Toledo, Escorial, Segovia). To each his own, they’re both good, but that’s my view. Madrid’s tapas bars are better also IMO.

@Lulu348. I meant that I like Madrid better the Barcelona. I would pick Seville and Madrid. Slate (response above) explains many of the reasons and I agree.

I recently returned from my first trip to Spain. Arrived in Madrid, took train to Seville for 3 nights, onto Cordoba for 2 nights, Granada for 3 nights and Madrid for 4 nights (and a day trip to Toledo). I loved my time in Spain. I wasn't sure I would enjoy Madrid, but I did and am happy I chose that over Barcelona.

My cities selections were based on which attractions I wanted to see. You may want to dig a bit more into what interests you and choose the city/day trips that appeal to you. There is so much to experience, but also make time for taking things slow and enjoying the experience.

It's all personal preference. Sevilla is a fantastic city but with that schedule I'd add another night to Barcelona instead.

Choose a direct flight rather than one through Madrid.