Ten days is the sweet spot for Barcelona, Madrid and Seville if you want more than a sequence of hotel lobbies and railway platforms. Barcelona gets three nights because Gaudí, the old city and the sea pull in different directions. Madrid gets three nights for the museum district, neighbourhood life and a deliberately unplanned evening. Seville gets three nights because southern rhythm is part of the point: the Alcázar and cathedral need booked time, but the city is at its best after the formal sightseeing is over. The train does the long work between them, leaving you to arrive in the centre rather than collect a car, navigate motorways and return it at every turn. Use the extra nights for neighbourhood meals and laundry, not another distant day trip; the route feels generous only when at least one afternoon in each city remains free of timed entries.