Barcelona -> San Sebastián -> Madrid for 17 days in October. Unsure how long to spend in each place.

3, maybe 4 days in San Sebastian is enough. By October spending time on the beach not a big draw, there are some things to do, leaving food (which is great). Then the balance in Madrid and Barcelona, I would probably leave more days for Madrid than Barcelona.

How do you plan to travel between the cities? Either by car or train, you will have some long travel days.

If you are going to day trip to Bilbao, I'd want to have three full days in San Sebastian. Another day if you'd like to visit Hondarribia, a beautiful seaside town close to San Sebastian with a castle of Charles V. San Sebastian has an excellent Basque museum.

In terms of Madrid and Barcelona, I'd watch some Rick Steves episodes and look at the site seeing opportunities in each city. Also look at possible day trip options from each. For Madrid, Segovia and Toledo come to mind. For Barcelona, Montserrate, Girona, and Figueres are possibilities. For me, personally, we were in Barcelona/Girona, about eight days and could have stayed more. It is a very unique city. I thought Madrid was very pretty and lively, but I think there are so many more historic and interesting cities in Spain than Madrid. They do have a lively food scene (I think all Spanish cities have great food, however), wonderful art museums and a lovely cathedral.

I´d spend at least two days in Bilbao (great urban planning, architecture, food, old quarter, Hanging Bridge, funicular, Azkuna Zentroa...), with more to see and do than in Donostia-San Sebastian. D-SS can be easily seen and enjoyed in two days, it´s a small city and not that many ammenities. Take into account that this is a rainy area any time of the year, and beach weather may not be an option in October.

This route seems to be coming up more.

Last month there was a similar question, Here is a link to that thread: https://community.ricksteves.com/travel-forum/spain/tips-d21599c3-4932-47aa-8084-2f0a60e71276

As Jules noted, how you travel is important. I suggest taking a train to Zaragoza. Spend a two nights there. If you have a car, you can go to Belchite. It was the scene of some fierce battles in the Civil War and they have left the town as is as a memorial.

How many days depends on what you want to see in each place. The sites in the places you seem to want to visit are not tourist traps. They are popular destinations for a reason. Determine what you want to see in each place along with how long you think it will take to see it. Total everything up and you’ll get an idea of the time you’ll need. Invariably you’ll spend more time at some places and less at others. Don’t visit a place just because somebody says “it’s must see.” Visit places of interest to you.

Belchite was mentioned as a side trip from Zaragoza. It is worth visiting but check out the times when tours are given. We missed the tour time, but fortunately we had a rental car and drove along the town’s fence line on a dirt path/road and we’re able to see everything; just not as close as we would have on the tour.

Other Spanish civil war sites near Zaragoza include, Monument to the fallen Posición de San Simón, Las Tres Huegas, Guerra Civil trenches, and Orwell Road (trenches, command center). They are all very near each other.

Outside of Madrid is yet another vestige of the civil war; the penal colony at Bustarviejo. It’s a civil war era penal colony for railroad workers a mile outside of town. It is best reached with a car.

Would it be advisable to add an extra leg after San Sebastian/Bilbao to Oveido, then to Madrid from there? I was thinking of exploring a town like Valladolid or Burgos after Basque Country en route to Madrid, but from what I've read, it doesn't seem like there's much to see/do there. I'm happy to be proven wrong though.

In all my trips to Spain, nothing about Valladolid drew me to it, so I’ve never gone. I’ve been by it on numerous trips. Burgos is very nice. Its cathedral, with the tomb of El Cid and his wife Jimena, is beautiful. Up the hill from the cathedral is the Mirador del Castillo that affords a great view of the city. Most things to see in the historic center are by the Plaza Mayor and Plaza Rey San Fernando. The Santa María arch is pretty and there’s a nice tree lined park that goes along the Río Arlanzón (more like a stream). A short walk away along the river is the monument to El Cid at Plaza Mio Cid. The Camino de Santiago goes right past the cathedral so you’ll probably see pilgrims in the plazas.

A few kilometers from town, but walkable is the Cartuja de Miraflores; a 14th century palace turned into a monastery. It’s the Royal Pantheon of the parents of Queen Isabella the Catholic, John II and Isabella of Portugal.

With a rental car there are a number of small villages worth visiting within an hour of Burgos.

Burgos makes a very good stop on the way from Bilbao to Madrid, its gothic cathedal is absolutely a must-see and its old quarter is very nice. The Museum of Human Evolution https://www.museoevolucionhumana.com/en/ (as nearby it´s Atapuerca, one of the most relevant prehistorical sites in the world, https://www.atapuerca.org/en/apartado/1989/sierra-de-atapuerca-sites

I totally agree with the others that Burgos is worth the visit.

Going to make a suggestion that instead of a leg after San Sebastian, a leg before. If you have a car leaving from Zaragoza, you could visit Huesca, Loarre castle and San Juan de la Peña, stopping in Jaca. Then cross the Pyrenees and go to Pau. Then head out to the coast, see Bayonne and San Juan De Luz before going to San Sebastian,

If you have a car, Asturias is fantastic to spend as long as possible.
Oviedo makes a good starting point.
I found that area more interesting than Burgos, as within easy reach of Oviedo you have mountains, coastal villages, some of the best restaurants in Spain.....

Valladolid (and environs) is an unsung gem...but probably off your true for this trip.

For food lovers, carnivores, that region would be in capital letters.