How long does it take to hike down the mountain?
From the very top of the peak down to Murren will be 4 hours: http://www.schilthorn.ch/en/hiking/
Joanne, When are you planning to do the hike? If you're starting at the top, part of the trail can be somewhat "precarious", especially if there's still snow on the ground. You could perhaps take the Cable Car to Birg and start the hike from there. Cheers!