It all depends on what you want to see. No one here can guess what you want to see/do/eat.
Look at a couple guide books and make a list of what you want to see and their hours. Then look at a map and see where they are in relation to each other. Make a draft itinerary and come back here. People will let you know if your itinerary is doable.
I've only been to Granada once but am coming back in the fall. Last time I spent 3 nts (2.5 days), this time I'm going 4 nts. You can see the sights fairly quick but, too me, its a super charming town. I'm spending my last nights in Spain before a return trip just to slow down a bit more, adding the extra night to just enjoy a few extra walks and yummy restaurants. (our fall trip we are flying thru Madrid, but hitting up Seville and Barcelona as well as Granada.). Happy travels!