Spain: how many days for various cities

3, 3, 3, 3, 5 and 2.
However, with 24 nights I would be tempted to do:

Four nights in Madrid with a day trip of your choice.
Four nights in Seville with a day trip to cordoba.
Three nights in Granada
Three nights in Valencia.
Seven nights in Barcelona with several day trips.
Two nights in girona.

Madrid - 5 nights with a possible daytrip to Toledo
Seville - 5 nights with a possible daytrip to Cordoba
Granada - 3 nights
Valencia - never been so don't know - maybe 2 or 3?
Barcelona - 6 nights
Girona - 2 nights

What are your entrance/exit airports?

Have you mapped out the things you want to actually do in each city? That would presumably help inform your decision.

If it were my holiday, I'd do it like this:

Madrid + 1 day trip (5 nights)
Sevilla + 1 day trip (5 nights)
Granada (3 nights)
Valencia + 1 day trip (4 nights)
Barcelona + 1 day trip (5 nights)
Girona + 1 day trip (3 nights)

Madrid 5 nights including day trips to Toledo and Segovia
Seville 5 nights including day trip to Cordoba
Granada 3 nights
Valencia 3 nights
Barcelona 5 nights
Girona 2 nights

That is 23 nights. Do not miss Toledo it is amazing.

Thank you for all the helpful replies!

@davebarnes

What are your entrance/exit airports?

It's actually quite complicated. I'm going to do a follow-up post to explain the entire trip. These six cities are what I want to cover on my own before, between, and after some organized tours I'm taking. Stay tuned for the details in another thread.

@Indyhiker

Have you mapped out the things you want to actually do in each city? That would presumably help inform your decision.

I haven't yet. It will be my first time in Spain, so I'm pretty much interested in the general highlights of each location. If you look at Rick's guidebook or https://www.ricksteves.com/europe/spain/, I'm probably interested in all the 3-triangle sights, most of the 2-triangle sights, and some of the 1-triangle sights.