There is a Matisse exhibition at the Alhambra between the 15th October 2011 and the 28th February 2011.
It is on the upstairs floor of the Carlos V palace which is opposite where you queue up to get into the Nazrid Palaces
It is free to get in although to avoid a crush there is somone limiting the amount of people who enter.
When I went we had to wait about 5 minutes to get in.
Mattisse visited the Alhambra on a short visit to Spain 100 years ago.
The visit had an influence on his later works, especially a piece of fabric made in the Alpujarras which he purchased in Madrid.
There are several works by Matisse on display and most people would take about 15-20 minutes to visit it.
I probably wouldn't have made a special trip just to see the exhibition as part of a trip to the Alhambra it is well worth seeing.