We hiked Ice Box Canyon last week and it was a great trip. Parking seemed a little tight and, considering we had to sneak in on the should of the road, I expected the trail to be crowded. Not the case at all -- I don't know where everyone was but the hike up only had us encounter maybe a dozen other couples.
It was a great late-morning hike because the constant shade makes for a cool visit. We saw a group climbing the steep walls to the north of the trail and, upon arriving at the waterfall, had plenty of space to relax.
I read some reviews that the hike is 2-3 hours. FYI, we weren't booking it but we were up in about 45min and back down in 40min (stuck behind a group of chatty cathy's).
Definitely a worthwhile stop on a Vegas trip.