It takes between 10 and 14 hours to hike Half Dome. The amount of time taken will depend on your individual health, activeness and the speed you use while hiking. See http://www.ehow.com/how_3694_hike-half-dome.html for more on hiking Half Dome.
Search for half dome online and it will possibly tell you the average time it takes to hike up it. Take lots of water and make sure you stretch first. I have never hiked it but my father has and I believe it takes a couple hours