Hot weather predicted for the weekend, if you go, take at least a gallon of water if you don't have a filter and are unwilling to drink from the Merced at Nevada Fall. If you are a 'dip and drink' person, still take at least a half gallon (more if it's hot) when leaving the Nevada Fall area, and drink your fill before leaving.
Generally 12 hours, 17 miles with nearly a mile of elevation gain (and a mile of descent), the last mile of which is very strenuous, at high altitude, and without shelter from the hot sun.
The hike: http://www.pbase.com/roberthouse/halfdome
Gary
Yosemite Photo Galleries: http://www.pbase.com/roberthouse/yo