In my opinion, Valley of Fire should be a full day trip so you can take time enjoy the scenery, visitor areas, etc.
Red Rock Canyon can easily be a half day trip, especially if the traffic on West Charleston is slow. I heard a rumor that as a result of the events of 9-11, they (BLM) was going to close the scenic loop off to cars and make you ride a tour bus. I haven't heard much lately about it although I suggest you do a little research before you commit. If you can drive your car and stop at various points for relaxation, photo ops, hikes, etc, it can make a very nice half day trip.