5-6 hours is doable, but on the fast side. I actually did it in slightly under 5, but I was solo and going really fast (like, probably faster than anyone I saw that day). Going down HT takes a lot longer than you'd think - the trail is steep and rough for much of the way. So yeah, I'd say plan on at least 6 if you want at least a little time to stop and enjoy the scenery, etc. Of course the rangers will probably tell you that it takes twice as long - but they tend to exaggerate things over there.