I've visited the Atomic Testing Musuem, the Mob Museum, and the Neon Boneyard. All are a welcome and fairly inexpensive way to take a break from winning in the casinos.
I'd allow at least three hours for the testing museum and the mob museum. I only had about 2 hours for the testing museum and missed seeing several things I'd like to have seen. It seems to me the boneyard tour only took about an hour, but I know they were hoping to expand and add a museum.
And the boneyard tour is outside, so like lasvegasgambler said, prepare for that.