I went up and back in about 4.5 - 5 hours. I kinda ran most of it though.
I think average might be around 10 hrs.
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Found this:
From:
http://www.yosemitevacation.com/hike.halfdome.htm "Yosemite National Park-Half Dome, the hike is approximately 17 miles round trip. The hike to Half Dome can take anywhere from 10-12 hours. Cables are put up in late May and taken down in October. The elevation will increase by 4,800', from Yosemite's Valley Floor, ( almost one mile )."