Madrid airport customs time

For Customs you walk through “Nothing to Declare”. Immigration, passport control will take longer. It is a huge airport,very long walks in several areas.
I would allow at least two hours, maybe more, unless in same terminal. Find out the terminals used for both flights.
We took a high speed train returning from Seville to Madrid, so easy, so that is an option unless you have already bought all your flights.

If you are gambling buying separate plane tickets then give yourself at least 4 hours. Other wise on one ticket just go with the airline minimum connection time where if you miss the flight to Seville they are obligated to put you on the next flight available.

I think it took us between 1-2 hours changing flights in Madrid. Both flights were in the same terminal. The airport has very good signage, but it's quite large.

The challenge in Madrid is that the relatively new, and certainly huge, international terminal building is a long distance (by bus) from the old domestic terminals. Some connecting flights do use the main terminal but not all. I suggest going to the airport website and checking the terminals against your flights.

If you are booking separate tickets there really is no “safe” time as stuff happens. If you are on all one ticket 2 hours will likely be OK and if not the airline needs to get you there. But, consider the train. You can take a train, taxi or bus to Atchoa station right from the airport. Then the high speed train is a pleasant 2 1/2 hours and you arrive in the center of Sevilla.

Looking at the flexible-ticket prices for tomorrow, it appears that the train from Madrid to Seville would cost no more than 76.30 euros per person. The train from the airport to downtown Madrid's Atocha Station would add a bit to the cost. There is a slight possibility that a given train might be sold out by the time you arrive in Madrid.

The ticket-buying situation at Atocha is less than ideal if you end up needing to use a staffed counter because the ticket-vending machines don't seem to like your US credit card (which is possible) or for some other reason. The staffed counters always seem to involve a really substantial wait, so if you decide to take the train to Seville, I suggest buying that ticket at the airport rail station so you're all set when you get to Atocha.

Atocha is a large and potentially confusing station, so unless you explicitly ask the clerk at the airport station to give you extra time, you shouldn't count on being able to search for food and toilets at Atocha before it's time to board. Best to get to the platform if the platform number has been posted when you get there. There's a brief security check before you get to the train, and I think the train doors may close a couple of minutes before scheduled departure time.