It would be lovely to be able to spend 5 days in each town we visit in Spain, but it isn't possible. And I've only been to Europe twice in 15 years, so there is no guarantee I'll be back in Spain very soon. That said, I'm going to see as much as I can while I'm there! (I did the same with Italy 5 years ago, and I remember it all very well and am glad I saw as much as I did.)
Anyway, our itinerary is for 15 days and also starts in Barcelona and ends in Madrid. We are flying to Barcelona & spending a few days. Then flying to Granada (to save a day of travel.) We'll spend a few days in Granada, and then rent a car. Spending 5 days in Andalucia (Nerja, Ronda - coast & mountains) then going to Seville for a few days. Then taking the AVE to Madrid to spend the last 4 days there. It is hectic, but we will make the most of our time, and get as much of the Spanish atmosphere as possible. One thing we do is pinpoint a few "must-sees" (like the Prado & the Alhambra) but not try to see every 'sight' and spend parts of each day just wandering around, sitting, eating, etc. That way, we get to see the sights that the city is famous for, and spend time soaking up the atmosphere.
Hopefully we'll be rich one day, and then we'll visit Europe (and everywhere) all the time. Then we'll have the luxury to 'sit back and relax.' But seeing places for a few days is better than not seeing them at all!