Leave Vegas early enough and you can do it in one day. We didn't bring lunch and had to leave the park to find food, so we lost a lot of time. But almost all the park is accessible by car, so you don't need to hike very much. A full day there should be enough (unless there are trails and areas I didn't see).

If you have a GPS, the backroads from the rear entrance from the park and back to Vegas are scenic, although if you leave the park and the sun is going down, you're best off sticking to the highway.