People line up way in advance for the Nasrid Palaces, and I'm not sure why. I did it since the line looked kind of long and in the end it didn't really matter much. 200 people enter in a steady flow. So the very first people in do get to see the first couple of rooms more or less empty. Most of what you're looking at is above head level so it's not a big deal. Those folks who want to be in first will be in line more than 30 minutes ahead. One of the staff began walking through the line about when the entry began to make sure that everyone had tickets for that time. If you get to the line after they've started going in, make sure the people ahead of you have the same time as you do. If you're more than a few minutes "late", go straight to the front of the line, assume everyone else is waiting for the entry after yours.
Once you are through the "magic" door, you can linger as long as you want in any room. There is a one-way route through the rooms but you can backtrack as much as you like, though you have to exit at the end of the route.
It is a long walk (downhill) from the main entrance to the Nasrid entrance. Since you probably won't go back that way, allow time to enjoy the vistas and minor sights along the path. If you want to visit the Charles V Palace, go early, it's usually (always?) only open in the morning. (It's been some 6 years or so since I was there, so possibly somethings have changed.) There is a multistory building near the entrances to the Nasrids and Alcazaba (also a ticketed entry). You can store large bags and backpacks (not allowed in the Nasrids), and there are toilets. If you store items, you'll have to backtrack after the Nasrid visit to collect them. If not, you may want to visit the Alcazaba before the Nasrid, so you don't have to backtrack for that. It doesn't take very long to see (maybe 30 min?) and there are nice views of the city below.
is very close to the entrance to the Nasrids. If you wait to