It's at least a 5 hour drive, and that's with no traffic, so I would recommend leaving Vegas at the crack of dawn. If there is an accident or just heavy traffic outside of Vegas, you could be stuck for hours!
You wouldn't think there would be much traffic out there in the desert, but the highway is only 2 lanes in each direction, so it can be slow going at times.
One time it took us almost 8 hours driving back to SD from Vegas on a Sunday in the middle of the day!